ctrl + alt + h on most ide's will list the callers of a function. It's pretty safe to repeat things and delete things, version control is a safety net.

I've done a lot of legacy work, and most repetition is due to a loss of business context. Naturally support staff don't want to be hero's and make big changes.

Right now I work with a system that's been in active development for 30 years and it's a dream to work on because everyone knows it and I can just ask some of the original authors what something is, or to explain an edge case.

Most dev's aren't that lucky.