Yes people do scoop one another, especially in very competitive fields. I have witnessed horror stories of a postdoc sending an advance copy of a paper in final stages of preparation to his former advisor, only to be "scooped" by the advisor who wrote a short comment on the same topic and submitted it to a journal with fast turnaround.
The reality is that citations and other forms of recognition (like who published first) are now the currency of the realm, whether we like it or not. It will come as no surprise to anyone from this forum to learn that there are now more people than ever looking for a slice of the funding pie and/or a faculty or research position, and some will seek any advantage they can to get ahead of the competition.
In the situation you describe, where the information is "out there" as an abstract or preprint or conference proceedings, it's considered public domain and all bets are off, although usually with proceedings or preprints there is a date stamp that allows to establish claims of who got there first. With just regular conference presentations or seminars, you're on your own and trusting you have an experience advantage over the audience so you can publish first.
(Indeed in some circumstances it's clear the work is far from completion and the authors just want to stake the idea as theirs.)