Reusing key/nonce affects security of CTR mode and Stream ciphers in general.
Assume that you have two cipher texts encrypted with the same key, say E(A) and E(B). The problem could be figured out mathematically as follows:

```
E(A) = key xor A
E(B) = key xor B
```

Now try XORing the two ciphertexts as follows

```
E(A) xor E(B) = key xor A XOR key xor B
= A xor key xor key xor B // algebraic property of xor
= A xor B // because key xor key yields 0, and XORing 0 with anything yields that thing
```

Given that A and B are normal English letters, the guessing of A and B will be trivial, as you lost the key space of the stream cipher. Now, you are just trying 26 letters.

The worst case scenario applies when A is longer than B, that means A xor B will blur only as much as B length, the rest of remaining bytes of A will be intact. Essentially, you have decrypted A just by XORing two cipher texts with any prior knowledge of the key and/or plaintext.

That mathematical fact is terrifying and tells you loudly _**Never REUSE the key**_