[202311] Handle learning duplicate IPs on different VRFs (#3165)

[theasianpianist]

202311 cherry-pick of https://github.com/sonic-net/sonic-swss/pull/3165

What I did Fixes https://github.com/sonic-net/sonic-buildimage/issues/18890

• If we try to learn an existing neighbor on a different VLAN in the same VRF, delete the old neighbor entry before creating the new one.
• For all other scenarios, proceed with neighbor learning normally.

Why I did it Allow learning the same IP in two different VRFs

How I verified it Run the C++ unit tests

Details if related

[keboliu]

@yxieca would you please merge?