Or, if you are talking about Texture Painting, maybe you Unwrapped the body while it still had a Mirror Modifier on it. Thus Unwrapping only one half. When you then Apply the Mirror Modifier, the left and right part of the body still have the same UV's, so paint strokes appear on both sides.
If that is the case, you'd need to Unwrap the body again after Applying the Mirror.
Thanks both,
Omar was spot on, found it in the UV editing tab in the end. Appreciate you both helping me!
