The following image shows when you should start a new note for your slipbox. And what to do if there are similar notes:

Note creation flow chart

Start by finding similar notes, if there aren’t any make a new note. If there are notes, but not related, make a new note.

Notes related to the topic exist, if these do not fit the idea create a new note but adhere to the keywords.

A few notes roughly match: creating a new zettel is a bit too much. In this case it is useful to compile the existing knowledge. Comment on the compilation and make links in both directions. Ignore the last line, backlinking is bad

When an exact match exists, you can simply extend what is already there.

This note is largely based on this documentation.