How to search for or avoid duplicate contributions for the same membership

Question

Imagine this scenario:

1. a person signs up in public membership sign up page
2. a membership and a contribution is created
3. the same person, a while later, forgets that they have sign up and sign up again in the same public membership page.
4. the current membership does not change (it's ok!) and a new contribution is created (mmmhh)

How can I inform the person that the membership has already been made?

Or, how do I search in CiviCRM for duplicate contributions for the same membership?

Update after Jay answer

When a contact pay twice the membership with the same email and in same period (year, fixed membership type plan in my own case) the membership is extended by a further 12 months. It would be optimal that the contact should be informed of the new end date and type of membership. How to do it?

To notify him and the organisation we could use CiviRules to send an email when "Membership is changed" trigger and "Membership End Date is changed". The problem is that the message template does not know which membership has changed, because it only has the 'latest membership' tokens...

Answer 1

Normally the membership gets extended by the membership period. Unless the user enters info that doesn't get caught by the duplicate contact check (typically configured as name and email). I occasionally get a problem where this happens when they use a different name or email (Bob/Robert is common!) and two contacts are created each with a separate membership. The contact merge (done manually) sometimes isn't able to correctly merge the two memberships (by adding together the remaining periods of each. I don't think it even tries). If its done automatically it always work properly for me.

Is this what you mean, i.e. there are two separate contacts with separate memberships? Are they using different contact info each time? You may want to review your settings for unsupervised duplicate contact (or is it one contact with two memberships?)

How can I inform the person that the membership has already been made?

If it is the issue above I have no idea. I just go and check and manually clean these up on occasion using other duplicate contact rules.

how do I search in CiviCRM for duplicate contributions for the same membership?

Not sure how to answer this. I tend to review recent contributions so always catch them. If I had to find them after perhaps running a report over a specific time period and then review those contacts that have multiple contributions?