Multiple failed saves from different ghasts

[Kershek]

When failing saves from multiple ghasts, do the stench penalties stack?

 

[Olgar Shiverstone]

How many other effects from the same source stack?

If Bob the wizard casts a bull's strength on you, and then Fred the wizard casts a bull's strength on you, do they stack?

 

[Kershek]

Ok, I get the point