This sounds patently absurd and I can imagine the cries
"But I know 2 + 2 = 4, what on earth could x + x equal?"Of course the point is that you don't know what x equals in this case, indeed the use of x is completely arbitrary. The letters in algebra are simply substitutions for unknown quantities,and we do do the same thing with x.
As we don't know what x means, our job is to either simplify a mathematical statement so that should we ever find out what x means it is easier to deal with, or solve an equation that will tell us what x is.