DuckDuckGo has a feature called bangs that lets you use sequences of characters staring with ! (in this case !so) to redirect your browser to that site's search page. For example, if you type in "toys !a", it will send you to an Amazon search for toys. This is not user-specific, and a "bang", as these sequences are called is present for every user of the site.
Every time that I'm not logged in (or at least in a browser that has not been logged in since a cookie clear / in a private window) and I make a search on a Stack Exchange site via DuckDuckGo, I am presented with a CAPTCHA. This is strange, however, as if I go directly to the URL of a search, I get no such CAPTCHA.
For example, if I search on DuckDuckGo
Is there an R function for finding the index of an element in a vector? !so
I get a CAPTCHA, but if I type directly into the address bar
https://stackoverflow.com/search?q=Is+there+an+R+function+for+finding+the+index+of+an+element+in+a+vector%3F
I do not.
Watching how DDG bangs work with the following curl command, we see that it is not an HTTP redirect, but presumably JavaScript:
curl -v -A 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:84.0) Gecko/20100101 Firefox/84.0' 'https://duckduckgo.com?q=Is%20there%20%20an%20R%20function%20for%finding%the%20index%20of%20an%20element%20in%20a%20vector \!so'
Result (relevant bit, and run in zsh):
< HTTP/2 200
It's kind of annoying that I have to fill out a CAPTCHA or log in every time I do this, even though it can be avoided by simply using the search bar on SO. Can something be done to prevent the CAPTCHA from being presented every time this happens?
Update
I wrongfully assumed that this was network-wide, hence why I asked meta SE. It appears that SO is much more likely to CAPTCHA me then AU is, presumably because it's a larger target for spam and crawlers. I've also noticed that having a referrer of stackoverflow.com seems to reduce the chance of a CAPTCHA, however I've done so much searching now to test that it seems to think I'm a robot every time I do anything on SO, and will have to continue testing once the system forgives me.