>The user control has a grid on it which apparently uses some javascript that includes the FindControl client side. The user control is placed on a page in a placeholder along with one or two other user controls. I am setting the ID of the user controls when they are added to the placeholder. The control is added something like this.
>I am not so sure the user control is the control which is the conflict. The control has a grid on it that causes the problem, but I can't tell what it is referencing that gives the message of multiple controls all have the control ID of ctl00.

I know I had issues w/adding a user control w/the Telerik grid. I ended up having to disable the viewstate on the grid control (I don't remember the specific problem I had though).

Are you dynamically changing anything about the grid itself (adding/removing columns, etc.)?

One other idea - after adding a control on each postback are you removing the control when it's not being used or if you are going to display a different control? You need to. Let's suppose you dynamically add control "A" the first time the page is hit. The user clicks some button which forces a postback and should now display control "B". In your page you reload control "A", then remove it. Now you dynamically add control "B".