Weaknesses arising from using same key in both channel directions
Suppose that the keys used in both directions are the same. You listen to something sent from A to B:

kjhsdfiugdsa8ltr634o87qy5843jlgrwjklfjgsfds


You then listen to something sent from B to A:

jh35i798fdgkjh53jkh87gfhkjsfhtjk35h1o8798ug


Does it matter that the key is the same? Can you understand any of the messages?

If the answer to these questions is negative, then eavesdropping can be ruled out.

In contrast, suppose that you sent the following message from B to A:

kjhsdfiugdsa8ltr634o87qy5843jlgrwjklfjgsfds


You don't necessarily have any idea what this means, but it is identical to a message sent from A to B.

Because the key is the same, whatever meaning the original message had, so does this one.

This kind of attack (which is easy to thwart even if the keys are the same) is known as replay.