By "unique combinations", I assume you mean unique rows.
Here is one way:
% Example dataset. (Use your data instead.)
x = [7 8;
1 3;
7 8;
1 2;
1 3;
1 3];
[uniqueRows,idxToUnique,idxFromUniqueBackToInput] = unique(x,'row','stable');
probabilityOfUniqueRows = histcounts(idxFromUniqueBackToInput,[1:max(idxFromUniqueBackToInput) Inf],'normalization','probability');
The output you need is uniqueRows and probabilityOfUniqueRows.
