I finally managed to reduce the testcase without hitting other bugs. 
This problem is caused by discovering a duplicate in the middle of 
reading in the entity in question.  I had thougt the import seeding at 
the beginning of a cluster prevented that, but it is insufficient. 
Specifically an earlier cluster in the same module can cause the import 
of a duplicate.  Although clusters within a module are well-ordered, 
there is no ordering between clusters of one module and clusters of 
another module.  And thus we can get duplicate declaration loops.  This 
prevents the problem by also seeding references to earlier clusters in 
the same module.  As the FIXME notes, it is sufficient to reference a 
single entity in any particular earlier cluster, plus, we also could 
determine the implicit dependencies and prune that seeding even further. 
  I do not do that -- it decrease the loading that will happen, but 
would reduce the serialization size.  As ever, let's get correctness first.

         PR c++/99283
         gcc/cp/
         * module.cc (trees_out::decl_node): Adjust importedness reference
         assert.
         (module_state::intercluster_seed): New.  Seed both imports and
         inter-cluster references.  Broken out of ...
         (module_state::write_cluster): ... here.  Call it.
         gcc/testsuite/
         * g++.dg/modules/pr99283-6.h: New.
         * g++.dg/modules/pr99283-6_a.H: New.
         * g++.dg/modules/pr99283-6_b.H: New.
         * g++.dg/modules/pr99283-6_c.C: New.
         * g++.dg/modules/hdr-init-1_c.C: Adjust scan.
         * g++.dg/modules/indirect-3_c.C: Adjust scan.
         * g++.dg/modules/indirect-4_c.C: Adjust scan.
         * g++.dg/modules/lambda-3_b.C: Adjust scan.
         * g++.dg/modules/late-ret-3_c.C: Adjust scan.
         * g++.dg/modules/pr99425-1_b.H: Adjust scan.
         * g++.dg/modules/pr99425-1_c.C: Adjust scan.

-- 
Nathan Sidwell