DeclareUnicodeCharacter breaks if called twice for same character

[Oliver Heimlich]

I have a manual for a free software package, which is written in utf-8.
Export to text/info/html works as expected, but I have to add the
following to be able to export it to pdf:

@tex
\DeclareUnicodeCharacter{00A0}{\ }
… several other characters …
@end tex

This has worked for me for a while, but it breaks on later Texinfo
versions with the following error:

/path/to/octave-interval/octave-interval/doc/manual.t2d/pdf/xtr/manual.texinfo:8:
Internal error, already defined: 00A0.
   \DeclareUnicodeCharacter ..., already defined: #1}
                                                     \fi \expandafter
\globalle...
   l.8 \DeclareUnicodeCharacter{00A0}{\ }


As far as I have found out, there have been the following relevant
changes to TexInfo in the meantime: (1) The character 00A0 has been
declared globally, which is great, and (2) commit 6086 has introduced
error checking, which forbids re-declarations for the same character.

Now I have the problem that I have to remove the DeclareUnicodeCharacter
commands, to be able to compile under recent TexInfo versions, but then
it is no longer possible to compile under old TexInfo versions (for
example in Debian Jessie).

Could you remove the above error message, or at least make it a warning
message? Then my TexInfo document would be backwards compatible.