Surely if you purchase a domain (domain1.com) and redirect it to your existing domain (domain2.com) you expect to loose all ranking for domain1.com. If 301's are used then the objective is simply to pass on authority, not dominate the search results with 2 domains. If you want to keep domain2.com in the index then you would take Rebecca's option 2 or 3.
2) prerelease auctions.  These are domains where the auction venue has contracted with the registrar, like Fabulous or Moniker, to auction off non-renewed domains.  Just before a domain enters the pending delete process, it goes to a private auction to any person who has backordered the name.  Some people feel that back links on these type of auctions still provide a SEO benifit. My testing indicates that there is zero SEO benifit when these domains are redirected into another site.  So if you get a domain with "pink squirels" in a lot of achor text to a domain and then point it at your domain, you don't rank for "pink squirels"
.ac .ae .af .ag .am .ar .as .at .au .aw .ax .be .bg .bi .bj .bn .bo .br .bw .by .bz .ca .cc .ch .ci .cl .cn .co .cr .cx .cz .de .dk .dm .do .ee .es .fi .fm .fo .fr .gd .gg .gi .gl .gs .gy .hk .hn .hr .ht .hu .id .ie .il .im .in .io .iq .ir .is .it .je .jp .ke .kg .ki .kr .ky .kz .la .lc .li .lt .lu .lv .ly .ma .md .me .mg .mk .mn .ms .mu .mx .nl .nc .nf .ng .no .nu .nz .om .pe .pf .pl .pm .pr .pt .pw .qa .re .ro .rs .ru .rw .sb .sc .se .sg .sh .si .sk .sm .sn .so .st .su .sx .tc .tf .th .tl .tn .to .tr .tv .tw .ua .ug .uk .us .uy .uz .vc .wf .yt .za
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