Hi Sciko, to add multiple instances of the same reference, use the named references feature. Taken from Wikipedia:
"For multiple use of the same inline citation or footnote, you can use the named references feature, choosing a name to identify the inline citation, and typing <ref name="name">text of the citation</ref>. Thereafter, the same named reference may be reused any number of times either before or after the defining use by typing just <ref name="name" />. The use of the slash before the > means that the tag is self-closing, and the </ref> used to close other references must not be used in addition. The text of the name can be almost anything—apart from being completely numeric. If spaces are used in the text of the name, the text must be placed within double quotes. Placing all named references within double quotes may be helpful to future editors who do not know that rule. To help with page maintenance, it is recommended that the text of the name have a connection to the inline citation or footnote, for example "author year page": <ref name="Smith 2005 p94">text of the citation</ref>.
Let me know if you still have questions. Thanks :)
