I’m facing a problem with a mesh that has a Multiresolution modifier applied. Everything was progressing well as I worked on the UV mapping. In an attempt to eliminate any duplicate vertices in the mesh, I opted to merge them by distance, which resulted in the merging of two vertices. Unfortunately, this action has caused an issue when viewing the mesh in Object mode, and it appears to be irreparable. Kindly review the images depicting the mesh before and after. Could someone please provide assistance with this?
Welcome 
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Changing the geometry after some use (adding and sculpting ) of multimesh modifier … is just a bad idea… if the underlaying mesh is changed… how do you think should the modifer do his work ??
(Also you mean added not applied !? Because then you just have a “simple” mesh… and any vertex merge will not alter the mesh it this way… but maybe the UVs 
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…or… you mean something completely different… then… :
You may have to elaborate this to get a more suited answer.