We use both the Lead and Customer entity during the sales cycle. When someone enters into our doors, digitally, they become a lead. This could be signing up for an in-person or virtual event, a newsletter, or filling out a lead gen form that then goes the business development team. Once that person becomes a sales-qualified lead, they are graduated to a contact. An opportunity may or may not be attached at this point, but for the sake of this example, it is not relevant. At this point, once they are converted to a contact, the lead record is set inactive. The issue then arises if that same individual was to fill out yet another form for another event they want to attend or perhaps they are signing up or updating their subscription preferences. This action creates yet another lead for them because all of our forms are set up as Leads, since we have to pick Leads or Contacts. So now I have a record of a contact and a record of a lead with the same email address. Data is no longer clean but moreover, because when we send out emails, we typically send them to leads and contacts. Since a Journey and a Segment cannot have both leads and contacts, we have to set them up separately. So this person will now get our email twice. Once as a lead, once as a contact. Deduplication doesn't work here because it only applies within the same Lead or Contact entity.

Not to solution this, but the ideal state here is either leads and contacts are both allowed to be part of a segment and journey, allowing for existing deduplication rules to apply here, or somehow the system needs to be an email match between leads and contacts, OR forms need to be aware of leads/contacts pre-existing with the same email address. That said, people could always upload a lead with the same email that a contact already exists with, so the issue isn't with forms alone.