I was emptying my legacy account today to send all the coins to a segwit account.
I realised that all the inputs sent to the same addresses (on legacy account) were than sent as separate inputs (although from the same addresses) to the new address. I wonder is this normal behaviour? It increased a lot already high fees. It would make sense to me that one address would be one input.
Is that just bad implementation of the wallet?
Inputs =/= Addresses. Every amount you receive separately is a new input. This means that if you receive 1 payment per day for 1 whole week, you will have 7 inputs at the end of the week, increasing the size of your transaction.