FetchResourceTemplatesByLabelSelector returns duplicate objects

[barbierajput378-pixel]

What happened:
The FetchResourceTemplatesByLabelSelector function in pkg/util/helper/binding.go
returns duplicate resource templates when the informer cache fails and the function
falls back to the API server.

What you expected to happen:
The function should return a deduplicated list of resource templates - either from
cache OR from API server fallback, not both combined.

How to reproduce it (as minimally and precisely as possible):

1. Open pkg/util/helper/binding.go (lines 336-383)
2. Look at the FetchResourceTemplatesByLabelSelector function
3. When objectList from cache is empty but not nil, and the function falls back
   to the API server
4. The function appends both the fallback results AND processes the empty cache list

Anything else we need to know?:
The bug is in the control flow logic:

• Line ~360: Gets objects from cache
• Line ~361-369: If cache fails/empty, uses API server fallback and appends results
• Line ~372-382: THEN it STILL loops through objectList and appends to objects
• This causes duplicates

This impacts any code that relies on unique resource templates from this function.

Environment:

• Karmada version: main/latest
• Others: pkg/util/helper/binding.go (lines 336-383)

[zhzhuang-zju]

Ref to #7671 (comment)
/remove-kind bug
/cleanup

[zhzhuang-zju]

/kind cleanup