Have mtk phone with corrupted IMEI no problem now you can do it easily within running phone as now MT65xx.apk can do the job easily. Just download apk put in sd card and install it tap to lunch and from interface you see it will ask for imei input for dual sim as well as single sim.
Download MTK65x.apk to fix rewrite imei of mtk china phones.
Note : Supports MT6577, MT6513, MT6515, MT6575, MT6517 are yet know but if it work on yours and not listed here tell us in comments and will add here to help other to find write tool.
Note : Phone must be rooted follow guide below to root mtk phone.