Basically, sometimes the user-agent will have its normal user-agent displayed, then at the end it will have teh "User-Agent: " tag displayed, and right after it another user-agent is shown. Sometimes, the second user-agent is just appended to the first one without the "User-Agent: " tag.

Here are some samples I've seen: The first few contain the "User-Agent: " tag in the middle somewhere, and I've changed its font to make it easier to to see.

Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0; Trident/4.0; GTB6; User-agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1); SLCC1; .NET CLR 2.0.50727; .NET CLR 3.0.04506)

Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; GTB6; MRA 5.10 (build 5339); User-agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1); .NET CLR 1.1.4322; .NET CLR 2.0.50727)

Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; User-agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1); .NET CLR 2.0.50727; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)

Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; User-agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1); .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.4506.2152)

Here are some without the "User-Agent: " tag in the middle, but just two user agents that seem stiched together.

Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.0; Trident/4.0; Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1); .NET CLR 3.5.30729)

Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; GTB6; IPMS/6568080A-04A5AD839A9; TCO_20090713170733; Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1); InfoPath.2)

Now, just to add a few notes to this. I understand that the "User-Agent: " tag is normally a header, and what follows a typical "User-Agent: " string sequence is the actual user agent that is sent to servers etc, but normally the "User-Agent: " string should not be part of the actual user agent, that is more like the pre-fix or a tag indicating that what follows will be the actual user agent.

Additionally, I may have thought, hey, these are just two user agents pasted together, but on closer inspection, you realize that they are not. On all of these dual user agent listings, if you look at the opening bracket "(" just before the "compatible" keyword, you realize the pair to that bracket ")" is actually at the very end, the end of the second user agent. So, the first user agents closing bracket ")" never occurs before the second user agent begins, it's always right at the end, and therefore, the second user agent is more like one of the features of the first user agent, like: "Trident/4.0" or "GTB6" etc etc...

The other thing to note that the second user agent is always MSIE 6.0 (Internet Explorer 6.0), interesting.

What I had initially thought was it's some sort of Virtual Machine displaying the browser in use & the browser that is installed, but then I thought, what'd be the point in that?

Finally, right now, I am thinking, it's probably soem sort of "Compatibility View" type thing, where even if MSIE 7.0 or 8.0 is installed, when my hypothetical the "Display In Internet Explorer 6.0" mode is turned on, the user agent changes to something like this. That being, IE 8.0 is installed, but is rendering everything as IE 6.0 would.

Is there or was there such a feature in Internet Explorer? Am I on to something here? What are your thoughts on this? If you have any other ideas, please feel free to let us know.

At the moment, I'm just trying to understand if these are valid User Agents, or if they are invalid. In a list of about 44,000 User Agents, I've seen this type of Dual User Agent about 400 times. I've closely inspected 40 of them, and every single one had MSIE 6.0 as the "second" user agent (and the first user agent a higher version of MSIE, such as 7 or 8). This was true for all except one, where both user agents were MSIE 8.0, here it is:

Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; Mozilla/4.0 (compatible; MSIE 8.0; Win32; GMX); GTB0.0)

This occured once in my 40 "close" inspections. I've estimated the 400 in 44,000 by taking a sample of the first 4,400 user agents, and finding 40 of these in the MSIE/Windows user agents, and extrapolated that to estimate 40. There were also similar things occuring for non MSIE user agents where there were two Mozilla's in one user agent, the non MSIE ones would probably add another 30% on top of the ones I've noted. I can show you samples of them if anyone would like.

There we have it, this is where I'm at, what do you guys think?