Combining multiple cells into one (without losing data) if the rest of the row is equal

Got it.

Here to regroup the years, i have used a simple if formula in sheet "Blad1", A1

=IF(OR(B2=1990,B2<1993),"1990-1992","")

Post how you want to regroup the years, we can modify the formula and get a list per the need. We can proceed from there.