I know this is a very old topic, but - you mention that you could have two talismans with the same ID and it would appear to work. In fact - I've found that when this happens, there are issues with both displaying the talisman and unlocking it, while I was working on preparing Lemmings Plus V for V12.7.
If you still have the level in question, can you see if on the latest version, you're still getting issues when the two talismans have different IDs? (Note that IDs only need to be unique within a level. You can have two talismans in different levels with the same ID, no problem there.)
EDIT: Flopsy mentioned on Discord that he doesn't even recall this topic, so unfortunately there isn't anywhere I can go with it from here. As such, I'm closing this topic. If anyone else has encountered a similar issue, please create a new topic to report it.