Fragmentation
Fragmentation in the wikisphere is the tendency for there to be many wikis with overlapping content. Sometimes this occurs by accident, due to people's lack of awareness of other wikis' existence, which leads to them to establish duplicative new wikis. Other times it happens when disgruntled users break away from existing wikis to form their own competing wiki (a process known as forking). Fragmentation is sometimes considered good, in that it allows for experimentation with different schemes of organization, governance, presentation, etc. It is sometimes considered bad, in that it increases the amount of time users have to spend checking various watchlists and otherwise performing the same work on multiple wikis.
Examples of deliberately fragmented communities include the original Wikitravel and the subsequent Wikivoyage; the former was originally an independent community, and the latter ended up joining the Wikimedia Foundation. For an example of the converse, see Enciclopedia Libre Universal en Español, which was spun off from Español Wikipedia.
Alternately, MeatballWiki was created as a space for talking about Ward's Wiki (the original wiki) and the larger wiki movement in general. Years later, WikiIndex was founded with a similar motivations: one being discussing and fostering community amongst various wikis. The two communities aren't in direct competition but have some overlapping scope and have developed distinct approaches.
See also federated wiki (concept).