two questions:
1. Is it a problem is the same R value is used for two different addresses?
There is a problem, if the addresses were generated with a hierarchically deterministic wallet, e.g. BIP32, without strengthening. This is only a problem if an attacker can also obtain the master public key. The master public key is usually not very well protected to enable simple watch-only access. E.g., electrum doesn't encrypt it, Trezor tells it without requiring the PIN.
2. How do you find reused R values?
I simply walk through the 30 GB block database and search for duplicates (with a self-written program).
I am using bitcoinj-0.11 for creating tx. Is this vulnerable?
Currently, I have not reused any address till now. However, I need to be sure I won't be affected by this.
Newer bitcoinj uses RFC6979. I think bitcoinj-0.11 used SecureRandom from the Java library to generate k. Although the name suggests that it is safe, this library function was unfortunately broken on some platforms, especially Android.
You can check if one of your addresses exhibited a duplicated R value here:
http://johoe.mooo.com/bitcoin/endangered.txt