I saw bridesmaid dresses I liked and then had some similar ones made because they didn't have them long enough for my sister in law.

The dressmaker insisted on them being slightly different, so I had it made of silk and organza instead of just silk, the straps were thicker and of a different material, and the flowers on the straps were different too. As they were made to measure, the shape on each was different too if that makes sense?

Not sure what the minimum number of differences is, but the dressmaker seemed happy with these.