A hypothetical question for you, SEN:

If someone is manipulating you, and you let it happen knowing this, and they think you don't know they're doing it, are they still manipulating you?

I don't think so. I feel that the definition of manipulation requires some element of deception.
Even so, if the manipulator has a backup plan that works even if you are aware of it, then ultimately you might still end up being manipulated.

I don't know. I voted no though.

Your hair stylist is manipulating your hair, and you are aware and allow it to happen, but that doesn't change the fact that your hair is manipulated.

I assume of course you're referring to psychological manipulation:
Your question essentially boils down to either: if you discover that someone is using underhanded techniques to influence your perception or behavior:

1) Is it still underhanded? (Yes)
2) Are you still influenced by it? (Yes/No)

The end goal of the manipulator is to get you to think a certain way, so if you think that certain way, they've accomplished their goal, whether it be from their manipulation or if it would have been your original intention. If your original intention has not been influenced by the manipulation, then you technically have not been manipulated. However, if you were influenced by the manipulator and later discovered it and did nothing, you're still a victim of manipulation.

In summary: having your thoughts or actions influenced through these manipulative tactics means you are being manipulated; knowledge of manipulation doesn't mean you aren't being manipulated. There's no "Other" option on your poll, so I must abstain due to the ambiguity of the question.

Though your hypothetical scenario is intriguing in that while you are (presumably) being manipulated, you are also manipulating the other person by making them believe you're oblivious to the situation.

Yes, because from their perception of things, they are still manipulating you. Honestly it's a matter of semantics, in the formal sense, and it's up to the individual.

He definitely is trying to manipulate you.
But unless he is actually swaying you towards his position / goal he's not successfully manipulating you.
It doesn't matter if you discover or not having been or being manipulated.

Commonly manipulation attempts are thwarted by the "victim" getting knowledge of it, but in other cases (e.g. doctor's recommendation, sexual coersion, etc.) they might accept being manipulated and follow through regardless. It's still manipulation then.

It can still be manipulation because you're manipulating the manipulator into thinking you're being manipulated, unless he knows you're manipulating him by letting him think his manipulations are manipulating you. However, the manipulator may still be manipulating if the manipulator knows that you're trying to manipulate the manipulator by pretending that the manipulator's manipulations are manipulating successfully. But you may still be trying to manipulate the manipulator if you are aware of these manipulations. So on and so forth, and this is why you should never play Diplomacy or Risk.

Never play Risk? kill the ! <3

Where is the pancake option?

What if the manipulator learns that the target knew about their manipulation all along? Would they see it as manipulation still?

As long as you were trying to get someone to do something without actually saying it, I would consider that manipulation. Even if the other person were to realise what you were doing it doesn't change the fact that you are.