There are lots of projects released under more than one license. One example is Mozilla’s tri-license, which states that the Software is released under the MPL, but the user can also choose GPL or LGPL instead of MPL (I think this is because the MPL license is incompatible with the GPL license).
However, I came across projects released under both the GPL license and the MIT license. What conditions must be met in this case? For the MPL/GPL/LGPL case, things are clear because you can choose MPL or GPL or LGPL. But from GPL & MIT I understand that the conditions from both licenses must be met at the same time in order to use it, and this doesn’t make much sense because they have different purposes.
In this case, should the MIT license be interpreted as an exception, similar to, for example, the QT’s LGPL exception or as an additional set of conditions on top of the GPL license?
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It means that you can pick either GPL or the MIT license (not both at the same time). If you pick GPL you need to go with the GPL terms (eg. all derivative works must also be released under GPL). If you pick MIT that’s not required (just an example among the many terms).