I hear "whole entire" used often. Shouldn't we use just one or the other in a sentence?
“Whole entire” is a tautology. The two words are synonymous, so putting them together adds nothing to the meaning.
Therefore yes, you should only use one of them. However if you hear the phrase spoken by someone else, I wouldn’t go as far as to label it incorrect. Grammatically, it’s fine to list multiple adjectives in a row. It’s just that in this case, it adds no further meaning.