As someone who prepares a new paper for each conference; I wonder if it is really a common practice to present the same paper at different conferences: I have heard the same paper at conferences in Europe an the USA within a year.
In addition to what Ivo said, it is the peer pressure, threat perception in getting singled out in academic community with respect to research output, physical number of publications, so called impact factors, research gate scored, hi-index, are perhaps encouraging this dangerous phenomenon. At times conferences are business units. However, some good conferences have rigours in managing processes. One cannot discount all. Besides individual ethics do count a lot. Self regulation is the best. Intent needs to be right, content would follow the right path if intent is pursued rightly.
Scientific conferences have several purposes, among them is to communicate new ideas, results, to propose new challenging problems to fellow researchers. The other purpose is to present long standing and well established theories in a refreshing, lively and creative way to larger and wider audience.
The last but not the minimal reason of conferences is to make a ritual of annual gathering (scientific pilgrim) so that thinkers, people of creativity, imagination and ideas from horizons of science meet, share and discuss, to connect the young and new comers to science research with the seasoned, the prominent and the established for future collateral works and thereby the advancement of science and its influences continues.
A conference is totally different from article publishing, which requires new results or a considerably modified older result where the changes are apparent.
Claus you are posting a very interesting point here and as it is usual it promotes different approaches all of them valid and complementary. The key point I will agree with Ivo and build upon Dejenie is the purpose of the Conference. The approach of Artur is absolutely valid in the dissemination-documentation continua of the research and learning programm instead the George and Claus point of view is absolutely valid in the strictly research endeavor domain (i.e. to systematic get and share new knowledge to a particular interested audience).
In the case of Conferences where Proceedings are edited after the Conference (no matter they are free of charge or public paid) it is not ethical (in Harekhrisna words) to present the same units of research, but in a scenario where it is not generating a 'publication' it is absolutely valid to use the same core ideas and build a better message in each presentation.
Finally I strongly believe that the long tradition conferences is some kind of friend's club where like an Oporto's wine you can mix old and established ideas and viewpoints with young researchers products and work in progress that changes the taste making it a very different and interesting mix. If all the attendees take the conference seriously then it will be a very fruitful space to get new ideas from young researchers while veterans improves novices with precise and constructive feedback.
Thank you all for your answers. I still believe that it is not correct to repeat an identical presentation at different conferences. Conferences should be for discussions of research. After that the presentation is made to a paper and will get published. There is no sense in "tourning" conferences with a single presentation. For the organizers of conferences it is becoming more and more problematic, since one cannot google every proposal. But I guess at least at annual conferences one should not simply repeat last year's presentation.
In my field, not every presentation at each conference is developing into a paper. Also, I normally attend different conferences of different commununities. So the emphasis of a talk might shift from conference to conference until a state is reached when finally a paper is published. For sure, a peer reviewed paper should only published once. And for sure, the presentation at a certain conference should not repeat the last presentation at this conference - even if it is a quadrennial conference