Ender dragon egg can be spawned twice.
This happened when I played on my private server for friends( official server without any patches/plugins, hosted myself on VPS hosting ). Players played from Windows and Android platforms.

I don't know when second dragon egg was spawned.
1. After the first dragon kill, I immediately collected the dragon egg and placed it in ender chest.
2. A few days later, I respawned a dragon using end crystals. Killing a dragon gave the same amount of experience as the first time.
3. Today I decided to create a dragon again to gain experience and saw the second egg...

Could it be a client-side chung generation bug?

Unfortunately, I can't reproduce this bug twice.