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You have what is called ACANTHOSIS NIGRICANS
,which appears as dark and thick rugouse skin over neck and armpits .This associated with obesity ,and in some cases with diabetes too.You should get your blood insulin levels checked .The only cream which can work is tretinoin
(apply at night) and the one of the latest medicine which has shown good result is metformin
orally.Metformin also helps to reduce weight.But for this you need to consult a good dermatologist
and take treatment.Always use a sunscreen when you step out in the sun.