This fixes Trac #15994.

Previously RdrName.gresToAvailInfo assumed that the input list
of GREs had no duplicates. I accidentally broke this precondition
in this refactoring:

    commit 6353efc7
    Date:   Thu Nov 22 14:48:05 2018 -0500

    Fix unused-import warnings

    This patch fixes a fairly long-standing bug (dating back to 2015) in
    RdrName.bestImport, namely

(There was an ASSERT, but it's usually switched off in stage2.  It
tripped when I switched stage2 assertions on.)

The fix is straightforward: account for dups in gresToAvailInfo.