I have a flat file and a table which the flat file populates. I covert the data type of all of the columns except one to float [DT-R4] from DT_STR because the destination table has float columns. This mapping executes fine without any errors and uploads all of the data. But then I want to see whether there are duplicate columns being inserted and introduced a lookup transformation. But it does not let me compare the [DR-R4] column with the relevant float column in the table. When I change the data type of the flat file columns to DT_R8, it lets me proceed to the end of the lookup wizard but gives the error "Error 41 Validation error. Data Flow GL_HISTORY: Lookup [2505]: input column "gl-appropriation-amt" (2748) has a datatype which cannot be joined on. Main_Package.dtsx " in the package design window. Any help would be greatly appreciated. Thanks in advance.

You can't use float for Lookup Task because it is not an exact datatype. You have to convert to decimal or int depending on the value.