You can (re)use this filter (the 'filter new items'-machine) to prevent duplicate dataflows. It installs a (userlimited) cache which is active between requests.
So let say we have this payload:
msg.payload = ["foo","bar"]
and we sent it twice to a 'debug'-machine using two button-presses on a 'inject'-machine
the output of the first mousepress will be:
["foo","bar"]
and the second mousepress:
[]
(because the message was seen before and stored in the cache)