Yes it is bad opsec to send to the same address.

If you think about it, if you send bitcoin *to* the same address twice, then you'll end up spending *from* the same address twice eventually.

But you don't need to spend twice from the same address for it to be bad opsec. The moment you add funds to an address twice, you link those funds. Now we know both sets of funds are owned or have been owned by the same person. That may not be enough to cause you trouble, but the game is to avoid giving anything away if you can.