The Supercup rules in England, Spain, Italy, Germany, etc, go along the following lines.
The Supercup is contested between the League champion and the Cup champion; if the same club wins both the League and the Cup, then the Supercup is contested between the League Champion and either the League runner-up or the Cup runner-up (depending on the country).
This format has always bothered me. Why don't winners of a League-Cup double get awarded an automatic Supercup? Winning both already recognizes them as the best club across domestic competitions, which (as I understand) was the original purpose of having a Supercup.
Edit after some research, it turns out that some national associations do award an automatic Supercup to a club that wins a League-Cup double (e.g., Albania and Croatia, among them). Perhaps the more prominent one in this respect is France. Auxerre won an automatic Supercup following their 1995-96 double; the same was to have happened in 2008 with Olympique Lyon, but FFF decided at the last moment to allow Bordeaux (League runner-up) to contest the Supercup. A similar situation could arise this year, given that PSG has already won the League and is the favorite to win the Cup final in a couple weeks from now.