'How to include different javascript scripts that have same variable and function names inside

how can I implement this scenario in javascript?

<script defer src="script01.js"></script> 
<script defer src="script02.js"></script>

Where script01.js and script02.js have inside duplicate variable names, functions with the same name, etc ..

PS: from script02.js I need to return parameters.

Thanks to all.

javascript include

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