Avoid infinite loop on duplicate keysalts

When duplicate suppression was requested, we would enter an
infinite loop upon encountering a duplicate entry, a bug
introduced in commit 0918990bf1d8560d74473fc0e41d08d433da1a15
and thus present in release 1.13.

Rework the conditional to avoid the loop, at the expense of
additional indentation for some of the code.

Ticket: 8038
tags: pullup
target_version: 1.13.1
Add tests for duplicate detection

There's not an easy way to test for infinite loops other than
making the test suite hang, unfortunately.