For the past few months I've also been experiencing this problem (multiple of the same item in the toolbox) as well as the slow loading time (toolbox initialisation) when I first open the toolbox each time I open VS2010.
After many hours of trawling the web and trying various things I've finally managed to fix it on my computer for my particular installation scenario, as well as possibly identify what is happening. In my case the problem was related to the WCF RIA Services Toolkit April 2011, so if you don't have this installed then this probably won't work for you unfortunately.
Here's what fixed it for me (as well as the testing I did to confirm it):
1) Uninstall all Telerik products (including any VS Extensions that you've installed via Extension Manager)
2) Uninstall WCF RIA Services Toolkit April 2011 (IMPORTANT)
3) Open up VS2010, open the toolbox, right-click and select "Reset Toolbox"
4) Close VS2010
5) Reinstall all Telerik products
6) Open up VS2010 and open the toolbox. This first time will take a while as the toolbox cache needs to be constructed.
8) Load a solution that uses Telerik controls (in my case it was an asp.net website) and confirm that the toolbox only has one of each item in it.
7) Close VS2010
8) Open up VS2010 and open the toolbox. It should be quick this time as the cache is already constructed.
10) Load a solution that uses Telerik controls (in my case it was an
asp.net website) and confirm that the toolbox only has one of each item
Why does this work?
Well, I'm speculating here but I think that each time a change is made to the toolbox (e.g. new control installation) VS2010 will process all the toolbox items and cache them locally for quick lookup the next time you open the toolbox. It appears that on successful construction of the cache a flag is probably set somewhere to say that the cache is up to date. However, if the cache construction process doesn't terminate successfully then this flag is never set and thus each time you open the toolbox VS2010 attempts to reconstruct the cache. This explains the slow first load of the toolbox on each start of VS2010. I suspect that somewhere in the logic for the Telerik toolbox installer there needs to be a check to see if the item is already in the cache (not sure if this is possible) as they keep getting added each time the cache is constructed.
But, I hear you say, what does this have to do with the WCF RIA Services Toolkit April 2011???
Well, during my trawling of the web I found this MS Connect thread
. Take a look at the Post by
MawashiKid on 26/06/2011. He successfully narrowed down a crash in the toolbox installer package to the WCF RIA Service Toolkit April 2011 package. It seems that this "rogue" is crashing the toolbox cache initialisation process and then the follow-on effect is for the toolbox cache to try and reconstruct itself each time...
At any rate, my advice to the testers at Telerik is to install WCF RIA Service Toolkit April 2011 and then try reproducing this issue. You might have better luck! Also, I would be interested to know how many of the people who are having this issue have WCF RIA Service Toolkit April 2011 installed on their machine?
Hopefully this will help people that are experiencing the same issue!