The error could appear if something was added to the page before it was supposed to.
Usually this is caused by calling DataBind too soon or .ClientID. This could happen if you are calling something that is not part of the page controls hierarchy yet - for instance calling.ClientID before OnPreRender. You can try to narrow down the issue using a clean page that does not uses and js, css and other controls and drag/drop controls one by one. You could also try using a "clean" control - just one Literal or Label declaratively added to the template of the control and noting in the code behind.
You could share with us steps you follow, so we could try to reproduce the problem locally and figure out what the problem is.
The quick fix to this issue would be setting the control ID from the code behind to a random value. You could use Random Class to generate random number and assign it to the ID of the control. This will ensure that each control has unique ID.
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