First, as I hinted at originally, this not intended to be production-quality. In fact, it is just how I would start a golf problem - get it working, then trim away.

runrig explained the how. The why (why add comma-space at end) is to make the whole string uniform (digit-comma-space, digit-comma-space, digit-comma-space, etc.) for easy matching.

The fun hasn't worn off yet, so here's some uglier code:

map{
   $m=!s/$/, /;
   $x=y/,//;
   for$n(1..$x){/^((?:\d+, ){$n})\1+$/&&{$m=$n,last}}
   print$/,($m>0?$x/$m:0);
}<DATA>
__DATA__
1, 2, 5, 1, 2, 5, 1, 2, 5
3, 6, 3, 6, 3, 6, 3, 6
4, 1, 28, 0, 4, 1, 28, 0
3, 5, 17, 3, 17, 5
4, 1, 28, 0, 4, 1, 28
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