What are compound words called that are composed of words with similar meaning?
You've asked an interesting question that the answer heavily depends on the example in your question.

When you see 磨蹭 as 'dillydally, stall', i.e. not a simple add-up of the characters, it is 联绵词 (binomes, see Baidu, Wikipedia). 联绵词 are words consist of two characters that are similar in pronunciation and/or meaning, but the meaning of the word is different from the individual characters (exceptions exist like 蝴蝶). You can find more information in the link. Some examples for verb binomes: 徘徊, 犹豫, 挣扎, etc. 

When you see 磨蹭 as 'to rub' i.e. rhetorical repetition, it is 同义复词 (synonym-compound, see Baidu). Note 复词 means 'phrase' instead of 'word', usually they are created on demand (although idiomatic 'set phrases' exist) and dictionary doesn't list them. This type of phrase has no grammatical significance over other types of phrase.