I'm not sure how this message is related to a multilaod file, because it sounds like you are trying to add periods here.
Can you post the header and the first line of your multiload file?
Maybe this will help you in the meantime: each period in FDM has a unique identifier which is the period in date format. If you are trying to add a period which already exists, then FDM gives this error and says: this period already exists, can't added because the criteria for the unique identifier would be violated.
Check which periods exist and what it says in your file header.