I have many duplicated wp_postmeta record (about hundreds record with same post_id & meta_key) and need to delete safely via mysql, is there any effective way to delete the postmeta record.
Further question is how to create code implementation to prevent this postmeta duplication. Many Thanks 
It is quite complicates grouped request, so to avoid problems, consider to use Cleanup Duplicate Meta plugin.
The reason why duplicate entries are occured is usage of add_post_meta(). Just use update_post_meta() (even to create meta, it works) – and you will have always one meta field with certain name per post.
