I think you're correct. The spell's effect is based on caster level. This implies, though it's not clearly stated, that at your specific level, this is the most you can increase the wind. Casting it again should not let you exceed the maximum you can achieve for your level.
Besides, at the minimum level you can cast this, you can already whip up 51mph winds on a calm day or whip up hurricanes from a moderately windy day. That's a lot. If you could exceed that, then any 9th level druid could flatten cities with tornadoes that last an hour and a half long - that's already more than 10x longer than the average real-world tornado, so think of it like a 9th level druid casting two spells to blast a city with (effectively) TEN tornadoes...
I believe so, yes. It would be the same as if two casters tried to cast the same light or dark spells; the stronger would win out but they wouldn't stack.