IMEI Number Mismatch After Repair

Hi Gurus,

My iPhone-6 (phone name - xyz iPhone) went under repair due to screen damage. As per the process they asked me to take back-up (took it on 11th May) and disable the "Find My iPhone" feature.

Now I've received my phone but I was surprised to see some changes :

1. Previous IMEI and new IMEI are not same.

2. IMEI on navigation(Setting -> General -> About) and at back of the iPhone are same but not the older one. While IMEI on pressing #06# is still the older one.

3. On navigation (Settings -> Apple ID), I can see two devices. One is older device (xyz iPhone) with older IMEI while other is new device (xyzabc Iphone) with new IMEI

4. When I connected to iTunes to restore my backup (from 11th May) I couldn't do. iTunes first asked me to update the IOS then took a whole new backup for device (xyzabc iPhone). Now I can't find my previous backup from (XYZ iPhone).

PS - On my repair bill, I can see fields like "New Serial Number", "Old Serial Number", "New IMEI", "Old IMEI". Also I verified through "Check Your Service and Support Coverage - Apple Support" and iTunes to confirm if device is genuine or not.

Plz advise -

1. If there are 2 IMEI, does it mean that hardware has been mishandled and security of my iPhone was compromised?

1. What is the disadvantage of having different IMEI number on handset to that of *#06#, as it can impact if I wish to sell my handset.

2. Should I keep both the devices as in point#3, or can I remove anyone - if yes then older device or new one?

3. How I can restore back-up taken from old device on this new device?

Plz let me know if you need any additional info.

Thanks,

Shivalik