The attached code example provides a simple example tests for the once only controller, possibly simulating a HTTP test where a login is required on the first loop by each thread.

The test would look like this:

Test Plan
+--Thread Group 
   +--Once Only Controller
      +--Task (e.g. Login)
   +--Task (repeated)
+--View Results Tree

In this construction the test plan operates as expected showing a single Login Task in the results tree for each thread, followed by multiple tasks from the repeated test section.

The attached file meets the structure above.

However if you now highlight the Once Only Controller and the repeated Task then select insert parent, adding a simple controller as hte parent the test results change.

Test Plan
+--Thread Group 
   +--Simple Controller
      +--Once Only Controller
         +--Task (e.g. Login)
      +--Task (repeated)
+--View Results Tree

Under the the new structure the test seems to repeat the Once Only Task twice for each thread.

I can see no logic behind this behaviour.