Member of List - does it check by email or a unique ID?

[Anthony_Pica]

One of our smart campaigns is set to add a lead to a list, but first checks to make sure that lead is not already in the list. 

http://screencast.com/t/NrJxUNiT -- does this filter check by the lead's email address or by a unique ID? I'm asking because I found duplicates in the same list. I was hoping this filter would prevent that by checking the email address, but if it's checking by a unique ID, then this filter won't work unfortunately.

We only want the lead to be able to be added to the list once, even if there are many duplicates of the same lead.

[Anonymous]

It looks at the unique ID. I had the same problem where I actually had to do manual deduping on Excel of the list that I exported from Marketo because I had leads which had the same email addresses but different IDs.