I also faced same problem and by doing a lot of Gooogling i found one solution to this problem.
You need things :
1. A PC, data cable and a phone.
2. MauiMETA 3G:*v6.1316.1 DO GOOLGE
3. BPLGU modem file for mt6582 DO GOOGLE
4.CDC Drivers DO GOOGLE
5. SP drivers Download Here: androidfilehost.com/?fid=23252070760972673
6. A brain.
1. Download all above, extract as needed.
2. Connect Phone to pc in usb debugging mode and install drivers.
3. For windows 8.1/8, move your mouse to top right corner and click on settings-power. now press and hold shift and click restart. from advance options select 7 to disable driver signing.
4. Once PC is restarted, press windows key+pause break to open system properties.
5. From left select device manager.
6. From actions menu click add legacy hardware.
7. Click next and select Install the hardware that i manually..... (advanced), click next, select show all devices, click next, click have disk, then browse, locate SP_Drivers folder and select cdc-acm.inf, click open, click ok, select MediaTEk Inc. and MediaTek USB VCOM (Android), Click next and next again, click finish.
8. Install MauiMETA and run it. Use USB COM and from Options menu select Connect smart phone in meta mode.
9. Press Reconnect and connect phone in switched off state with battery. The blinking led will become yellow and you can see "connected with target"
10. Now from drop down menu select IMEI download. Click ok till you see IMEI download box.
11. Click Change NVRAM Database FIle and browse to MT6582 DB File\MT6582\BPLGUInfoCustomAppSrcP_MT6582_S00_MOLY _WR8_W1315_MD_WG_MP_V5_1_wg_n (BPLGU modem file for mt6582). Click ok on any warning.
12. Leave IMEI Increasemnt 0. Write first 14 digits of your IMEI in IMIE box and last 15th digit will go in check sum(should be entered automatically). Eg:- if IMEI is 123456789123456, write 12345678912345 in imei and 6 in checksum. Do same for Sim_2
13. Press Download To Flash. You will see Download IMEI to flash successfully. Now close the box (or alt+f4 or close from task manager if stuck).
14. Restart the phone, see the magic. It will fix Your IMEI problem permanently even after factory reset.