In times of stress or unease, you speak in tongues. Pick one of the following languages: Abyssal, Aklo, Aquan, Auran, Celestial, Ignan, Infernal, or Terran. Whenever you are in combat, you can only speak and understand the selected language. This does not interfere with spellcasting, but it does apply to spells that are language dependent. You gain the selected language as a bonus language. At 5th level, pick an additional language to speak in combat and add it to your list of known languages. At 10th level, you can understand any spoken language, as if under the effects of tongues, even during combat. At 15th level, you can speak and understand any language, but your speech is still restricted during combat.
When a character takes their first level of Oracle and chooses the Tongues Curse, can they pick a language they already know, for example an Aasimar picking Celestial?
The argument I see against this would be something like 'You cannot gain a language as a bonus language if you already know it.'
If not, does that mean a character who already knows all of these langauges (via ranks in Linguistics, etc.) is unable to choose the Tongues Curse?
It does not state you have to pick a language you do not know so there is no reason you cannot pick one. If you do so you don’t gain a bonus language to replace it. This is no different than any other time you gain an ability you already have. Also keep in mind that you can only speak the languages you pick as part of the curse in combat even if you already know the others. So even if you know all those languages when you become an oracle you can only speak one. At 5th, 10th and 15th level you can pick another language to understand in combat. Not sure why anyone would pick a curse they get no benefit from, but it is not against the rules.