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For one of my apps, I want to make the source code “open source” so that others might adapt the code to their personal needs or even contribute improvements, but I don’t want anybody else to publish “my app”. Furthermore, I still want to be able to sell the app at some time in the future.
Is there a usual license for this, like the GPL or similar?
I found similar questions from 2010 which say something about dual licensing, but maybe there is a license fitting my needs?
If you don't allow re-distribution of your software, then your code is not "open source".
To my knowledge, there is only one 'license' that forbids redistribution of the original software, and that is the default option of copyright law: no license.
Personal modification, without redistribution, is (to my knowledge) allowed under the copyright law in most jurisdictions, and if it isn't there is no effective way to enforce it anyway (apart from withholding the source code).
First off, how big is your checkbook?
Putting a copyright notice on your stuff is one thing. Enforcing your copyright is something else. When (not if but WHEN) some scumbag steals your stuff, you're going to have to pay a lawyer to go after said scumbag. This costs money.
Also note: In the US, for all practical purposes, to enforce a copyright, you must have registered the copyright with the government. This costs a small amount of money. The lawyer costs a lot more. The money you pay for the lawyer will be WASTED if you didn't pay money to register the copyright FIRST. This is the on-the-ground reality in the US. Deal with it.
Having gotten all that out of the way...
The simplest answer is this: Put proper copyright notices on your stuff, with the magic incantation "All Rights Reserved", and put it on your website or FTP server or wherever. Anyone is free to read copyrighted stuff. Anyone is free to modify THEIR OWN PERSONAL COPY of copyrighted stuff. (If this were not true, you would not be allowed to underline or highlight important stuff in your textbooks.) The line is drawn at COPYING and redistributing the copyrighted stuff.
The thing is this: Once you put it out there, someone WILL attempt to rip it off and sell it, regardless of what your copyright notice says. The world is full of scumbags, and not all of them are where your lawyers can reach.
