Wikipedia:Not a new editor

Editors who have been banned or blocked may return to the collaborative project under the guise of being a new editor. If you run into such an editor add them to your watchlist and negate all dubious edits they make. When you have gathered evidence to substantiate your claims that they aren't a new user, but are an editor evading their block or ban in order to protect the integrity of the collaborative project the most common and effective approach is to file a report at WP:SPI, or you may email CheckUsers checkuser-en-wp@wikipedia.org with cogent evidence or, if there is private or off-wiki evidence, contact ArbCom at arbcom-en@wikimedia.org.

How to spot them

 * They are polite and patient and can make strong policy based arguments with you under few weeks or few months of them joining the collaborative project and can do so in a collegial and friendly manner. They are also often in the habit of making intentional NEWBIE mistakes.
 * Within weeks of registering they are grounded in policy, are rude and condescending to veteran editors, are passive aggressive, make sarcastic comments, argue policy with you in a subliminal manner, they almost want you to be aware of the fact that they predate your existence as an editor.
 * Can create a very decent article with just 1 edit.
 * Their first edit being to the talk page of a sysop to request un-deletion of an article that got deleted prior them joining the collaborative project.
 * Are familiar with our markup.
 * Undue Boldness.
 * They know about bite and always cite bite when interacting with them. See WP:CITEBITE.
 * Whilst relatively new can work with templates.
 * Fluent in wiki-lingo/Jargon despite being new.
 * They appear to know what they are doing and know their way around.
 * Know to always use their edit summary even in their very first edit.
 * Say things which imply a history predating (your) current interaction with them. For instance, an editor might make a comment like Foo is getting better at editing Wikipedia even though they have not interacted with Foo previously.
 * Within their first few days, they know what ANI is, and often threaten editors and system operators with reporting them to ANI when they are being corrected.

Alternate possibilities
Assuming good faith is a cornerstone of Wikipedia's culture, there are other reasons for newer users to be familiar with Wikipedia that are not nefarious:
 * They may have edited as an IP user without an account for months or even years before deciding to register.
 * They may be a legitimate clean start account or a valid alternate account used for editing certain sensitive topics.
 * They may just be quicker to learn things than others.
 * They may have prior editing experience on sister projects or on other websites that use MediaWiki.