The duplicate enum check returns false-positives

[volsa]

Describe the bug
These false-positives slipped in with commit e2e8fb3, take for example

TYPE
    State : (idle, running);
END_TYPE


FUNCTION main : DINT
    VAR
        stateA : State;
        stateB : State;
    END_VAR
END_FUNCTION

will report

error[E004]: State.idle: Duplicate symbol.
  ┌─ target/demo.st:2:14
  │
2 │     State : (idle, running);
  │              ^^^^ State.idle: Duplicate symbol.

error[E004]: State.idle: Duplicate symbol.
  ┌─ target/demo.st:2:14
  │
2 │     State : (idle, running);
  │              ^^^^ State.idle: Duplicate symbol.

error[E004]: State.running: Duplicate symbol.
  ┌─ target/demo.st:2:20
  │
2 │     State : (idle, running);
  │                    ^^^^^^^ State.running: Duplicate symbol.

error[E004]: State.running: Duplicate symbol.
  ┌─ target/demo.st:2:20
  │
2 │     State : (idle, running);
  │                    ^^^^^^^ State.running: Duplicate symbol.

which happens because get_enum_variants_in_pou will be called twice in the given example for the State enum, which should only happen once

Expected behavior
No duplicate error

Additional context
Will be much easier to implement once we refactor how enums are stored in the index