[BUG] Stack environment variables don't check duplicate names (overwritting issue)

Stack environment variables can be overwritten without warning/error.

When manually creating a new Environment variable for a stack (UI), it’s possible to create a new variable with the same name of an already existing one. When saving, there’s no error or warning about duplicate names, and the value of the existing variable gets overwritten. The toast notification says successfully created.

This behaviour should only be allowed by editing the existing variable. If a new variable has the same name of an existing one, the creation process should fail.