"Double letters" means two in a row, like tt in "letter" or oo, kk, and ee in "bookkeeper".
yak yellow yodoling yelling
Aardvark and eel are words. They begin with double letters.
Raccoon is a word. It includes double letters in a row.
coffeetoffeecommittee
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Llama
successfully
bookkeeper
accessaccommodationsbrassinessbeefinessembarrasscoffeechildlessnesscarelessnesscoolnesscheerfullycrisscrossclasslesscommitteecommittedfussinessfootlessfearlessnessfootloosefoolishnesshellishnesshopelessnessheaddresshappinessjoblessnesslawlessnessmoonlessmessinesspuffinessprissinesspeevishnesspennilessraccoonruthlessnesssuccesssleeplesssloppinesssleevelessspookinessstuffinessstiffnessspeechlesssillinessscruffinesstoothlesstattootoffee
Hawaii
fettuccinehiccuppingoccurrencesuccessfulsuccessive