In Yisro it says:
וְגַם בְּךָ יַאֲמִינוּ לְעוֹלָם
Earlier it says:
ויאמינו בה' ובמשה עבדו
Rashi (Shemot 19:9) says that G-d is telling Moshe that the Jews will also believe the prophets who come after Moshe.
The Ba'er Mayim Chaim explains that Rashi is saying this because the Jews already believed in Moshe, as stated in Shemot 14:31.
The SMAG answers we need both because they came to believe the first time when it says: ויאמינו בה' ובמשה עבדו
That was because of Miracles they witnessed and that is not a good way to establish a true belief as people can be fooled.But the Posuk Here:
הִנֵּה אָנֹכִי בָּא אֵלֶיךָ בְּעַב הֶעָנָן בַּעֲבוּר יִשְׁמַע הָעָם בְּדַבְּרִי עִמָּךְ וְגַם־בְּךָ יַאֲמִינוּ
We saw Hashem speaking to Moshe that is Harder to do Therefore we need the second Posuk in case a false Navi arises and does some tricks we can tell him that ia nice but Moshe WE WITNESSED SPEAKING TO GOD if you can do that we will belive you too.