I had the same problem here! The member wasn't being saved if i entered the same email as an existing member, even though I was using username as the unique value and there was 2 entries for each of the password and username being entered...

I grabbed the source and edited the file: SaveAsUmbracoMember.cs

 if (mt != null)
            {

                Member m = Member.MakeNew(nameMapping, loginMapping, emailMapping, mt, new umbraco.BusinessLogic.User(0));
                m.Password = passwordMapping;

...

Became...

 if (mt != null && Membership.FindUsersByName(nameMapping).Count == 0)
            {

                Member m = Member.MakeNew(nameMapping, mt, new umbraco.BusinessLogic.User(0));

                m.LoginName = loginMapping;
                m.Password = passwordMapping;
                m.Email = emailMapping;

...

This has solved the issue for me, though I'm not 100% sure why. I also added a catch by counting the Membership.FindUsersByName(nameMapping) to ensure no double ups.

Also while on it VS gives warnings that the Member.MakeNew method is obsolete, but if I create the member with: 

MembershipUser mu = Membership.CreateUser(loginMapping,passwordMapping);

How do I then get this new member as a umbraco.cms.businesslogic.member.Member to set the GenericProperties ?

Any thoughts?

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