  On Stack Overflow, users reuse 11,926,354 external links to share the
resources hosted outside the Stack Overflow website. The external links connect
to the existing programming-related knowledge and extend the crowdsourced
knowledge on Stack Overflow. Some of the external links, so-called as repeated
external links, can be shared for multiple times. We observe that 82.5% of the
link sharing activities (i.e., sharing links in any question, answer, or
comment) on Stack Overflow share external resources, and 57.0% of the
occurrences of the external links are sharing the repeated external links.
However, it is still unclear what types of external resources are repeatedly
shared. To help users manage their knowledge, we wish to investigate the
characteristics of the repeated external links in knowledge sharing on Stack
Overflow. In this paper, we analyze the repeated external links on Stack
Overflow. We observe that external links that point to the text resources
(hosted in documentation websites, tutorial websites, etc.) are repeatedly
shared the most. We observe that: 1) different users repeatedly share the same
knowledge in the form of repeated external links, thus increasing the
maintenance effort of knowledge (e.g., update invalid links in multiple posts),
2) the same users can repeatedly share the external links for the purpose of
promotion, and 3) external links can point to webpages with an overload of
information that is difficult for users to retrieve relevant information. Our
findings provide insights to Stack Overflow moderators and researchers. For
example, we encourage Stack Overflow to centrally manage the commonly occurring
knowledge in the form of repeated external links in order to better maintain
the crowdsourced knowledge on Stack Overflow.
