The two-deck example
Enter 7C@1,7C@2 and 7D. The inputs identify three different physical cards. The two club cards are nevertheless the same rank and suit, so the natural set fails this profile’s different-suit requirement.
Contrast the input error
Enter 7C and 7C@1 instead. These name the same physical card because a missing suffix defaults to deck 1. That is rejected before arrangement checking, and the page clears the previous explanation.
Make a valid nearby example
Change the second club to 7H. With 7C7D7H there are three different suits of the same rank. The checker can label it a natural set.
Do not transfer a special rule silently
Some other Rummy-family or multi-deck formats may have different allowed groups. This tool does not add those rules merely because two deck copies are available in the input.