Causes of Error 35602: Key is not unique in collection

When attempting to simulate or compile a PLD file on WinCupl, I get the error:

Error '35602':

Key is not unique in collection.

after which the program crashes and closes.

What is the meaning of an error like this? A quick search brings up many questions (e.g., this, this, or this), but these all yield application-specific solutions. My scenario is also application-specific, but WinCupl is an old program and I can't find similar support online.

Some of the answers to those questions say something along the lines of: "This problem is caused by a key in a table of the database that is not unique." Can someone give some insight on what this means, and/or possible solutions to look into?

_{Note: I was thinking about posting on Electronics Stack Exchange, but this program doesn't seem to be commonly asked about on that site and I thought I might get a better understanding of the error here. If you think a different Stack Exchange site is better suited for this question, please flag/comment.}

Solution 1:

I had the same same problem and traced it back to a point where the tree in the project window is populated . I was able to work around it unticking Project under View on the Menu before compiling. Not sure if this will work for you but I spent hours looking for a solution to this so figured I'd share my experience.