parshanut torah comment - Why does Moshe tell Hashem that he is 'heavy of mouth and heavy of tongue' - twice?

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)

Answer

[A friend of mine suggested this answer to me and it seems right: (no source though)]

The dialogue in Shemot 4:10-15 deals with Moshe's apprehension in speaking with the Jewish people.

Similarly Hashem tells Moshe in Passuk 16 that Aharon will be his interpreter- to the Jewish people. Shemot 4:16:

And he will speak for you to the people, and it will be that he will be your speaker, and you will be his leader.

So , until instructed explicitly, it would follow that Moshe would be speaking straight to Pharoh ( without Aharon as interpreter) which is why Moshe tells Hashem in Shemot 6:12 that he is "of closed lips".