Yes, you can put duplicate products in via an import but you have to do the following:
1 - the product reference for the duplicate should be in the format of x!yyy, where 'x' is the instance of the duplicate (ie the first duplicate will be '1', the 2nd will be '2' etc). 'yyy' is the orignal product reference. So the first duplicate of product '1234' will be 1!1234. You should also note that as references cannot be more than 20 characters long, original product references should not be more than 18 characters if using duplicates as you need the two extra characters in duplicate references.
2 - you will need a column for 'Original Product Reference' which needs to be mapped during the import
3 - you should have a column for 'Product Type', where '0' is a standard product and '1' is a duplicate.
1 - the product reference for the duplicate should be in the format of x!yyy, where 'x' is the instance of the duplicate (ie the first duplicate will be '1', the 2nd will be '2' etc). 'yyy' is the orignal product reference. So the first duplicate of product '1234' will be 1!1234. You should also note that as references cannot be more than 20 characters long, original product references should not be more than 18 characters if using duplicates as you need the two extra characters in duplicate references.
2 - you will need a column for 'Original Product Reference' which needs to be mapped during the import
3 - you should have a column for 'Product Type', where '0' is a standard product and '1' is a duplicate.