group_by_levenshtein.rb

emails = Customer.select(:email).map(&:email)

emails.map do |email|
  grouping = emails
              .group_by{|em| Levenshtein.distance(email, em) }
              .select  {|k,v| k < 5 && != 0 } # ignore exact matches and ones far off
              
  if !grouping.empty?
    [email, grouping]
  end
end

Assuming that the following is desired:

email | matches
-----------------
abc@test.com | [acb@test.com, cab@test.com]

My guess is that the actual query would look something like this:

SELECT email,
ARRAY(SELECT
b.email
FROM customers b
WHERE b.email != email
AND levenshtein(email, b.email) < 5
AND levenshtein(email, b.email) > 0) as matches
FROM customers

Then an AR version of it would be

Customer.select(:email).select("ARRAY(SELECT
b.email
FROM customers b
WHERE b.email != email
AND levenshtein(email, b.email) < 5
AND levenshtein(email, b.email) > 0) as matches
")