Function already exists

I insert callback function .onUserAbort in my skript but when I try to compile the last one, compiler says that the function is alredy exists, however I didnt used such function before.
Am I dumb? 🙂 Please help.

Modern UI uses this function itself.

Use:
!define MUI_CUSTOMFUNCTION_ABORT function
To create your own.

-Stu

Thanks a lot!!!