Is it valid to have duplicate Execution Modes

Came across this while writing validation for https://github.com/KhronosGroup/SPIRV-Tools/pull/4828

I assume the following is not actually allowed, but `spirv-val` doesn't check it and nothing in spec to refer to it being illegal.
```swift
OpExecutionMode %entrypoint LocalSize 1 2 1
OpExecutionMode %entrypoint LocalSize 1 1 2
```

Start with the duplicate OpExecutionMode examples in this issue and the referenced SPIRV-Tools pull request #4828. Check the SPIR-V specification for rules governing repeated execution modes, then clarify whether the example is valid and update the relevant specification text or validation guidance if needed.

Written by the indexing model from the issue text.