Fix duplicated index entries and avoid them in the future by adding a test.

[JJ]

When doing make

• found duplicate index entry is (inheritance) at #i45 #i679
• found duplicate index entry :{} at #i853 #i855
• found duplicate index entry compunit at #i276 #i277 #i278
• found duplicate index entry declarator regex at #i258 #i594
• found duplicate index entry increment operator at #i361 #i365
• found duplicate index entry control flow at #i53 #i54 #i55 #i56 #i58 #i60 #i61 #i65 #i68 #i69 #i71 #i74 #i75 #i76 #i77 #i78 #i79 #i80 #i81
• found duplicate index entry decrement operator at #i363 #i366
• found duplicate index entry compunit at #i273 #i274 #i275
• found duplicate index entry sym at #i260 #i263
• found duplicate index entry ++ at #i360 #i364
• found duplicate index entry whenever at #i50 #i51
• found duplicate index entry NFG at #i214 #i851
• found duplicate index entry «=» at #i430 #i765
• found duplicate index entry ∅ at #i628 #i733
• found duplicate index entry prefix & at #i129 #i871
• found duplicate index entry $*USAGE at #i147 #i836
• found duplicate index entry invocant at #i39 #i498

Among the gaggle of errors and warnings that pod2onebigpage emits. That would need a separate issue, or just point to #1823.
This error is also found in #1910

[JJ]

For instance, the first:

$ grep X doc/*/*.pod6 | grep inheritance
doc/Language/classtut.pod6:X<|is (inheritance)>
doc/Language/typesystem.pod6:The L<trait|/type/Sub#Traits> X<C<is>|is (inheritance)> accepts a type object to be

There should be some test for this, I guess...

[tisonkun]

Eliminates :{} duplicate 0a92a89

[JJ]

Duplicate index entries are back with a vengeance. We should not only eliminate them, but add a test so that they don't happen again. And in order to do that we need clear indexing specs, which we don't now.