Mirror sites are one of the biggest pains for editors, close to being spam. The question is: What is a mirror?

The suggestion guidelines say

• Do not submit mirror sites. Mirror sites are sites that contain identical content, but have altogether different URLs.
• Do not submit URLs that contain only the same or similar content as other sites you may have listed in the directory. Sites with overlapping and repetitive content are not helpful to users of the directory. Multiple submissions of the same or related sites may result in the exclusion and/or deletion of those and all affiliated sites.

So technically a mirror site is one that contains identical content. Pure mirrors are ones that are exactly, dot for dot, letter for letter, the same as another site. In many cases, there are mirrors that are almost identical, the layout may be slightly different, but the wording is 99% the same.

The editing guidelines on identical mirrors say Sometimes identical mirrors are created to ease the load on the main site. Try to determine which of the mirrors appears to be the original site, and list that one, if it is not already listed. Then remove the remaining mirrors.

In many cases, there are mirrors that are similar, the layout will be different, the wording is different, but it’s substantially the same site. In some case, one site may emphasize a single product shown on the other site. ODP calls these fraternal mirrors. The editing guidelines {see Affiliate Reseller Sites (aka Fraternal Mirrors} say

Fraternal Mirrors have the same basic content, but usually different designs. Fraternal mirrors are harder to spot because the sites are designed and written to appear different, but a careful examination will reveal they offer the exact same product or service as another affiliated company. They are usually set up by merchants as affiliate, reseller, or lead generator sites. For example, sites which sell products or services provided by another company and make a small margin on the sale are affiliate mirrors. In general we do not list affiliate sites unless the affiliate has strong, high quality content of its own that end-users will find really useful.

The bottom line is that suggestions of mirror sites are a very bad idea, they make an editor question your motives, and can substantially delay the review of a site.