Syntactically they are the same. 地 following adverb can be omitted in spoken language and in many cases, written language too. In ordinary cases, the omission does not change the meaning or the tone of the phrase.
In your example, however, the meanings are different. As the other answer suggested,
慢慢地来 is just an ordinary phrase meaning
come slowly; proceed slowly, mostly used in narratives, e.g.
他慢慢地来了。He came slowly.
Note the above example sounds lack a point. In real life usage, it is more natural to specify the action verb, e.g.
老人扶着拐杖，慢慢(地)走过来了。Supporting himself with a cane, the old man was slowly coming over.
On the other hand,
慢慢来 is always used for its metaphorical meaning
take your time; take it easy. It's mostly used as an imperative instead of narrative:
没关系，慢慢来。Don't worry, take your time.
慢慢来，小心点。Take it slowly and be careful! (as when someone is moving an antique for you)
There is a situation where 慢慢来 is used in a narrative sentence and interchangeable with 慢慢地来. They are used as adjective or adverbial phrase as opposed to predicative phrase. Note how the following examples are narratives from 3rd person view, although you can use them to make suggestions to the listener just like using imperative sentences.
感情的事要慢慢(地)来。Love cannot be forced.
这种类型的工作要慢慢(地)来。This kind of job takes time and patience.