Psalm 51/Notes/Lexical.vv. 4-5.179946
From Psalms: Layer by Layer
If הַרְבֵּה/הֶרֶב is read as an imperative (or infinitive absolute), then it means to do an activity multiple times. In conjunction (i.e., hendiadys) with כבס, it means roughly "wash a lot/thoroughly." If הַרְבֵּה is read as an adverb, it modifies the verb with essentially the same intensifying meaning "wash thoroughly." The sense in context is not repeated iterations of distinct washing events, but rather repeated action within a single, thorough washing event.