Why Membership and Other Amount doubles receipt amount
That definitely sounds like a bug.  It's not really replicable on the demo server, because you can't get emailed receipts, but if you can replicate this on a site built with buildkit, you can file this at https://issues.civicrm.org.

To start tracking the bug down, I'd insert the {debug} Smarty token into your receipt.  Hopefully there's an obviously wrong variable.