As per the documentation (https://book.civicrm.org/user/current/membership/defining-memberships/) :
"If you wish to enable online sign ups or renewals, the CiviCRM data model requires that a contact can only have one active membership with a single organisation at any given time. "
I currently have 2 CiviCRMs/Drupal : one in production, and one for tests (both in 7.44/4.7.6), and I use Webform to create the memberships.
The test Civi does not actually create duplicate memberships : each new membership replaces the previous one : which is in accordance, and correct.
How can this happen ? Is this connected with "If you wish to enable online sign ups or renewals" ? How can I change that ?