Shemot 4:10-15 is a dialogue between Hashem and Moshe where Moshe tells Hashem that he is 'heavy of mouth and heavy of tongue'.
In pssukim 14-15 Hashem tells Moshe that he will tell Aharon what to say, and that he will be Moshe's interpreter, because of his speech impediment. [see Rashi there]
So why then does Moshe say the same claim in Shemot 6:12
... How then will Pharaoh hearken to me, seeing that I am of closed lips?"
to which Hashem answers seemingly the same answer in Shemot 7:1-2 (ie that Aharon will act as interperetor)