Dapper uses normalization for the type fields, and this can cause problems when the fields are not unique. Recomendation is to use a uniquelly normalized name for each field, which would not conflict with other normalized fields names.

Bad:

public class MyType
{
    public string _firstName;
    public string first_name;
}

Good:

public class MyType
{
    public string firstName;
    public string firstNaming;
}