When/how can one be funded by multiple grants? Many researchers are funded by multiple grants. I want to understand how this works, at least in general. 


Can one be funded by multiple grants for the same (duplicate) work? 


I'm guessing not, but perhaps there are circumstances that allow it?

Can one be funded by multiple grants for the same project in different proportions?  


E.g. Grant A pays for half, and Grant B covers the other half, together paying a full salary. 

Can one be funded by multiple grants for different aspects of the same project? 
Can one be funded by multiple grants for independent projects?  


I'm guessing this is fairly common....

Are there grants that specify exclusivity?  


E.g. "You must focus on this project and may not receive any external funding for any other projects until this project is completed."



I ask multiple questions here because they have a common answer, such that if I asked them separately, they would almost certainly be marked as duplicates.
What I have seen the most is that the you can have two related projects funded by different grants. They are complementary, but virtually the same techniques are used for both. Officially some people would say that they are different projects and require different funding. Furthermore, you could have different students working on the different projects. In reality, the research aspect of the project (not the results) is the same. For example, you students working in different projects would need the same background and reading material. 

A made-up example can be machine learning. One project can be "solving traffic with machine learning", the other could be "predicting traffic with machine learning". The problems/results/solutions can be presented differently, but the core of the research could be the same.