Running a game and noticed the Soresu Circle of Defense Talent adds +X Defense X =☆ successes however I do recall a rule that you only take the highest Defense.

So if you already have armour with Defense 1 and something like cover or this gives you "+2" Defense do you have 2 or 3 Defense?

Different sources stack.
Talents and armor/cover stack.

Cover and armor do not stack.

At least that is my understanding.

EDIT: My mistake, Defensive Circle doesn't increase defense, it just gives a static Defense value to choose from.

So in your example, you would have 2 Defense - because you would be choosing between the static 1 Defense from Armor or the static 2 Defense from Defensive Circle.

Things that specifically say "+X to Defense" (like Hunter's Sixth Sense talent) or have Defensive/Deflection item qualities (like Cortosis Shield) which increases defense are different from static Defenses, because these get added onto the multiple static defense sources you may or may not have. When it says "gains" it means that you basically have an extra source of static defense choose from.

So for example, in ranged combat you've got Armored Clothing (gain 1 Defense), you're behind cover (gain 1 Defense), and you've got a Personal Deflector Shield (from EotE; gain 2 Defense), and you've got a Cortosis Shield (increase Melee Defense by 1, increase Ranged Defense by 2), and the Sixth Sense Talent (+1 Ranged Defense) and you have your Lightsaber to use Defensive Circle and you roll 1 success and 4 Advantage which is spent to increase the defense it gives you (gain 3 Defense). So first you'd pick the highest static defense - which would be the Armor, the Cover, the Deflector Shield, or the Defensive Circle action - which will be the Defensive Circle's 3 Defense. Then you add on everything that increases the static defense, which would be +1 from Sixth Sense, and +2 from the Cortosis Shield. Then you add everything together which results in 6 Defense.

tl;dr "Gains" in relation to defense gives you a different static option to choose from, "increase" (which isn't what Defensive Circle does) adds defense onto the static option.

Thanks. I didn't want to misinterpretate it and penalise my players.

General rule of thumb I've found is that if the defense value doesn't have the plus symbol in front of the number, then it generally doesn't stack with similar types of defense.

Just to clarify, if a PC wears robes with a Defense of 1, then carries a weapon with Defense 1 they stack to defense 2 right? The defensive entry clearly mentions it adds to it.

Just to clarify, if a PC wears robes with a Defense of 1, then carries a weapon with Defense 1 they stack to defense 2 right? The defensive entry clearly mentions it adds to it.

Yes, as the Defensive quality is one of those "add +1 to melee defense" so it'd stack with the defense bonus from armored clothing, providing the PC with Melee Defense 2.