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

Bad:

public class MyType
{
    public string First_Name { get; set; }
    public string FirstName { get; set; }
}

Good:

public class MyType
{
    public string FirstName { get; set; }
    public string FirstNaming { get; set; }
}