Our suggestion is using a where clause when you populate the table. You can expand the "where" area by clicking the little arrow to the far right of the table.
For each [row] in [some data] where [not [the same table you're already in] contains [some unique value in the data]]
e.g., if i'm creating a table of user data, and i'm looping over the response from a "get users in group" i might do a table with the name "mydata", and so the clause would be like:
"for each [user] in [the response to my Get Users in Group step] where [not "mydata" table contains the current user's UID]"
Here's a snippet of what it could end up looking like also: