Avoid duplicates in Silverpop

Description

Upon form submission, duplicate leads can be created in the 4th step below.
This scenario involves four visitors to a site and three browsers.

1) Visitor1 submits the form from browser1 (Chrome) with email address Visitor1@example.com.

Expected: New lead will be created (Visitor1@example.com).
Actual: New lead will be created (Visitor1@example.com).

2) Visitor2 submits the form from browser2 (Firefox) with email address Visitor2@example.com.

Expected: New lead will be created (Visitor2@example.com).
Actual: New lead will be created (Visitor2@example.com).

3) Visitor3 submits the form from browser3 (IE or Safari) with email address Visitor1@example.com.

Expected: Email already exists in the system from #1, so that lead gets updated. No new lead here.
Actual: Email already exists in the system from #1, so that lead gets updated. No new lead here.

4) Visitor4 submits the form from browser1 (Chrome) with email address Visitor2@example.com.

Expected: Email already exists in the system from #2, so that lead gets updated. No new lead here.
Actual (this is the bug): It will end up in a “duplicate” record, Visitor2@example.com.