I'm completely new when it comes to making mods from scratch (only edited existing mods before). So, I made a simple dungeon in the CK, but now - every time I try to load the .esp into the CK, I get an error message about duplicate form IDs.

FORMS: Duplicate form ID (01000D62) encountered in file 'testmoddungeon01.esp'.

Pressing 'Yes' gives me another warning.

FORMS: Assigning duplicate ID form new ID (01001420).

If I press 'Yes' again, the program crashes ('Creation Kit has stopped working' etc). Pressing no to either of the warnings opens a window asking if I want to exit the program.
Any suggestions?

I would also like to know how to change FormID's if that's possible. I'm making a bunch of new spells, and having problems getting them working. Possibly due to a similar issue.

Actually got it to work by deleting the form ID entry in TESSnip. The entry was a NPC from a mod containing ONLY this NPC that I loaded while making the dungeon. Weird, as I hadn't placed the character in the dungeon at all.