An imac retina 27" i5 (model 15.1) comes with 8GB (2X4GB) of RAM.
I am purchasing 2x8GB chipsets.
Will I cause a problem with my imac if I just add the 2x8GB sets yielding 24GB, or must I remove the original 8GB (2X4) first as apple suggests.
I just did the same thing the other day. Works fine. One thing I would note is that the memory that came preinstalled was in slots 1 and 3 so I just put the new memory in 2 and 4. It wouldn't startup! So I removed the memory and tried again. Put the original 2 x 4gb into slots 1 and 2 and the new 2 x 8gb into slots 3 and 4. All worked fine and have had no problems since.
You won't have any problem if you add more memory. But, be careful, new memory must have the exact same specs as original.