mattur - 3:39 pm on Jul 5, 2010 (gmt 0)
No, and it shouldn't ever be necessary to do so either.
This line is called the Document Type Declaration [en.wikipedia.org]. It is only used by browsers to switch between "standards mode" and "quirks mode" for rendering, and by some tools like validators.
A web page can only have one doctype declaration. Adding more than one means you're relying on browsers' error correction routines, and it would be nonsensical, since a page can't be, for example, both HTML and XHTML, or both loose and strict.
The one you choose should reflect the syntax you use on your pages. See Choosing the best doctype for your site [webmasterworld.com]
We recently had some misc programs or toolbars which did not work right. After contacting support they said it was due to the W3 lines (or lack thereof) at the very top of the index page.
Are your problems occurring due to your pages defaulting to "quirks mode" (i.e. no doctype) when external tools require "standards mode" i.e. layout issues (?)
If so the solution is to add one, appropriate, "standards mode" doctype. HTH.