When breaking the Nether portal part in creative the breaking sound (Glass) plays two times. When breaking the obsidian frame the breaking sound only plays once. My best guess is that a sound is created when you break the specific nether portal block, the game updates, then a sound is played for the other nether portal blocks that break.

Expected Results:

The Nether portal breaking sound only plays once

Actual Results:

The Nether portal breaking sound plays twice

Steps to reproduce:

1. Create a Nether Portal and light it
2. Break the purple swirling part in creative not the obsidian frame