The diacritics on "بِالْلَّيْل" seem off here, or is it really "biaalllayl"?
1. If you are talking about the Alif in الـ (or the definite article that is), this Alif is always hamzat wassl (ironically, despite its name, there is no Hamza ء above this Alif). Hamzatul Wassl acts a bit like a schwa and its sound is connected to what is preceding it (almost vanishing completely).
2. The Shaddah on ل is natural. Actually, in my answer I didn't use any diacritics (this is how we usually write and type in daily life, i was Just explaining the sequence of letters). Probably Duolingo did not explain this concept yet, but in Arabic and by the nature of letters, there 14 letters that are dubbed as "Lunar letters" and another 14 dubbed as "Solar" letters". Not to go deep, it is a phonetic necessity. The solar letters (which including L ل) are letters that require the merge of the (L) in (AL) into the first letter of the word and forming double letter (Shaddah). Example: شمس (šams) -> الشّمس (aš-šams) [sun -> the sun]. Notice that I'm typing the pronunciation here in my transliteration and not a letter-to-letter depiction. Experiment with your tongue, and try to ALSHAMS, I'm sure you can say it, but with certain difficulty, this is because the distance that the tongue has to travel inside the mouth to change positions from saying "L" to "SH" is relatively long. Thus, it naturally comes to us that the position is kept at "SH" and doubled or stressed as ASH-SHAMS. This is what we have here with اللّيل by nature since the letter itself here is doubled in fact: AL-LAYL -> ALLAYL.
Bonus: Maltese in fact uses the same system but since it is written in Latin, they do change the spelling of the definite article, unlike in Arabic where the spelling of the word and the definite article is not changed. Example in Maltese:
qamar (moon) -> il-qamar (the moon).
xemx (sun) -> ix-xemx (the sun).
check it out with Google Translate.