Double summon is important card to have in many decks and specially in gemini deck since they are normal until  summoned once again in field also you could use it to summon another monster to field in case your opponent wiped the field   by card like torrential tribute or mirror force etc .....

A combo could come in handy in your first turn by first summoning double tribute monster like kaiser sea horse then activate double summon to summon high level monster like  blue eyes white dragon or dark magician to make it hard for your opponent and heading trap that prevent the summoning like solemn warning or torrential tribute and if you summon dark magician you could activate card that need hi to be in field like thousand knives