Irs asked me to complete form 6198 to support some investments not at risk. This was already done on my 2018 return. Should I redo this? It was completed on my return

did you efile - it should have been included.  however,  while a waste of postage, I see no reason not to send a copy to the IRS provided the notice is really from the IRS.   other option call 800-829-1040 and when you speak to agent inquire as to why they need a copy when it was with efiling.