Examples in Postgres and CockroachDB showing that the st_union aggregate function does not ignore duplicates. The number of duplicate values input to the function affects the output.

st_union.crdb.sql

-- 1 duplicate.
SELECT st_union(g) FROM (
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
  FROM generate_series(1, 1)
) gen(g);
--                                                                                                                                                                                                                                                                     st_union
-- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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
-- (1 row)

-- 2 duplicates.
SELECT st_union(g) FROM (
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
  FROM generate_series(1, 2)
) gen(g);
--                                                                                                                                                                                                                                                                     st_union
-- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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
-- (1 row)

-- 5 duplicates.
SELECT st_union(g) FROM (
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
  FROM generate_series(1, 5)
) gen(g);
--                                                                                                                                                                                                                                                                     st_union
-- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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
-- (1 row)

-- 10 duplicates.
SELECT st_union(g) FROM (
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
  FROM generate_series(1, 10)
) gen(g);
--                                                                                                                                                                                                                                                                     st_union
-- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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
-- (1 row)

st_union.postgres.sql

-- 1 duplicate.
SELECT st_union(g) FROM (
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
  FROM generate_series(1, 1)
) gen(g);
--                                                                                                                                                                                                                                                                     st_union
-- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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
-- (1 row)

-- 2 duplicates.
SELECT st_union(g) FROM (
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
  FROM generate_series(1, 2)
) gen(g);
--                                                                                                                                                                                                                                                                     st_union
-- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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
-- (1 row)

-- 5 duplicates.
SELECT st_union(g) FROM (
  SELECT '01060000000200000001030000000100000004000000000000000000444000000000000044400000000000003440000000000080464000000000008046400000000000003E4000000000000044400000000000004440010300000002000000060000000000000000003440000000000080414000000000000024400000000000003E40000000000000244000000000000024400000000000003E4000000000000014400000000000804640000000000000344000000000000034400000000000804140040000000000000000003E40000000000000344000000000000034400000000000002E40000000000000344000000000000039400000000000003E400000000000003440'::GEOMETRY
  FROM generate_series(1, 5)
) gen(g);
--                                                                                                                                                                                                                                                                     st_union
-- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
--  010600000002000000010300000002000000060000000000000000003E4000000000000014400000000000002440000000000000244000000000000024400000000000003E4000000000000034400000000000804140000000000080464000000000000034400000000000003E4000000000000014400400000000000000000034400000000000002E400000000000003E4000000000000034400000000000003440000000000000394000000000000034400000000000002E4001030000000100000004000000000000000000344000000000008046400000000000004440000000000000444000000000008046400000000000003E4000000000000034400000000000804640
-- (1 row)

-- 10 duplicates.
SELECT st_union(g) FROM (
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
  FROM generate_series(1, 5)
) gen(g);
--                                                                                                                                                                                                                                                                     st_union
-- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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
-- (1 row)

NOTE: It's easiest to tell the difference in the values by looking at the last ~3 characters of the return value.

Postgres version 14.9
Postgis version 3.3.4

CRDB version v23.2.0-alpha.4-dev