Hi Sara Martínez de la Hidalga,
Usually this problem arises when you try to upload the same bank statement is repeatedly imported at the same time. If possible try to make some changes in the bank statement with regard to statement number, etc and upload.
I believe you might have been having this issue for your test system. For Production system, generally it will be uploaded only once.
So, try to make some changes in statement and upload again.
Regards,
Sridhar