editing community posts results in non-community-post mesage

[Former user]

editing a community posts such that the hashkey changes to that of an existing community post yields the error message "You already have this publication in your collection."

This should be changed to "There already exists a community posts with the data you provided. Please use the existing post"

[Stephan Doerfel]

fixing #2392

→ <<cset 437c66023adf>>