Yosemite keeps make a new desktop space

[basanova]

Hello,

Using 21.5 iMac Late 2013, Yosemite has been clean installed.
I also use 24 Thunderbolt display as its 2nd display.
The problem is, Yosemite always making unwanted 2nd desktop (desktop 2) on its primary monitor.
I can close it manually but it's so annoying.

Is there anyone who experience same issue or resolve this issue?

Thank you.