וַיִּוָּלֵ֤ד לַֽחֲנוֹךְ֙ אֶת־עִירָ֔ד וְעִירָ֕ד יָלַ֖ד אֶת־מְחֽוּיָאֵ֑ל
וּמְחִיָּיאֵ֗ל יָלַד֙ אֶת־מְת֣וּשָׁאֵ֔ל וּמְתֽוּשָׁאֵ֖ל יָלַ֥ד
And Irad was born to Enoch, and Irad begot Mehujael, and Mehijael
begot Methushael, and Methushael begot Lemech.
Rav Hirsch explains that this is because מחויאל (in his youth) became מחייאל as a result of the declining morals of his generation. That is from a passive resistance to Hashem, the generation turned into an active resistance. The extra Yud in the name according to Rav Hirsch is to show this change and make the distinction.
Irad begat Mechujael, מחוי אל in which godlines was extinguished. When
the generation was young, it was מחויאל, passive, godliness was
extinguished int, but later when it was older it was מחייאל, active,
tried to do away with godliness. It was followed by מתושאל (מתו the
root of מתים), seeking masses of people.
Note that the transliteration would show the two words as Mechu-yael turning into Mechi-yael. Thus it is not really a double yud but the vowel on the chet is חִירִק while the second yud is the consonent.