I ran into this issue today and couldn't find a great solution online.

Needed to combine multiple key/value files, like .ini or .env, and have some values override others while removing duplicates.

This script + example demonstrates how to use awk to remove duplicate lines based on their key.

What is your expected merged.env content? Lines from which file take precedence? Listing it will help understanding your use case.

specific overrides base. top down.

Could be whatever way you want it to be really, but that's the case here.