Indexing some notions twice?

Question

I'm making an Index of defined notions for my thesis (it's partly algebraic so there are dozens of notions defined through the whole text) and I think on how to index notions like trace of a Mobius transformation or side of a fundamental domain.

The nature says that the main part is the noun so it should be

fundamental domain
   Ford
   pre-Ford
Mobius transformation
   rational
   real
side
   of fundamental domain
trace
   of Mobius transformation
vertex
   of fundamental domain

But then it could be indexed the other way as well, because both Mobius transformation and fundamental domain are in Index anyways.

fundamental domain
   Ford
   pre-Ford
   side of
   vertex of
Mobius transformation
   rational
   real
   trace of

My question is whether it is confusing to merge both variants and make such terms indexed twice.

fundamental domain
   Ford
   pre-Ford
   side of
   vertex of
Mobius transformation
   rational
   real
   trace of
side
   of fundamental domain
trace
   of Mobius transformation
vertex
   of fundamental domain

Answer 1

This is, also, just an opinion, but I feel like you should indeed make the index as complete as possible, since you don't know under which term the reader will be looking for a certain expression. I have no idea what your notions are about but if the reader (i.e. in the context) can be expecting them to be filed under two names, go for both.

However, you should not index the same thing twice under a different name. Decide on the one you want to use, and make another entry that just says "see [the first entry]". For instance:

Fundamental domain: ..., side of, 5,6,8,12,35, ...

Side: ..., of fundamental domain (see under Fundamental domain), ...

Answer 2

Well, this is an opinion, nothing more. I think that the main purpose of an index is to help the reader of your thesis to locate things quickly and easily. There are many ways to achieve this but I guess that certain crossrefs in the index like

foo page 14,
 see also bar

bar page 14

can be helpful, in particular if foo and bar have several subitems. It is clearly an art to make a good index ;) But just keep in mind the reader: what kind of pre-knowledge can you assume, what kind of notions might he be interested in, after what keywords might he search? So "second-countable" might appear as standalone word, but also as subitem of "topological space" as well as of "manifold".