The Samsung Galaxy S4 mini (http://www.gsmarena.com/samsung_i9190_galaxy_s4_mini-5375.php) in its dual-SIM incarnation (I9192) is already on sale from multiple online stores in multiple countries.
Amazon US carries the Galaxy S4 mini dual-SIM at $483, but there are only 20 units available at that price and only of the Black Mist (http://www.amazon.com/Samsung-Galaxy-Gt-i9192-Black-Unlocked/dp/B00DRNEV9S/ref=sr_1_1?s=electronics&ie=UTF8&qid=1374231178&sr=1-1&keywords=samsung+galaxy+s4+mini+dual+sim) color version. If those run out or you prefer White Frost (http://www.amazon.com/Samsung-Galaxy-Gt-i9192-White-Unlocked/dp/B00DRNAT7G/ref=sr_1_2?s=electronics&ie=UTF8&qid=1374231178&sr=1-2&keywords=samsung+galaxy+s4+mini+dual+sim), there are 10 units at $489.
Cells4U (http://www.cells4u.com/Samsung-I9192-Galaxy-S4-Mini-Black-Unlocked-p/sgh-i9192-bk.htm) undercuts price to $470 for the Black Mist version, cheaper than the single-SIM version.
Then there's Techno Trading House (http://www.technotradinghouse.com/samsung-i9192-galaxy-s4-mini-duos-dual-sim-black-unlocked/) at $515, available only in black. It's $5 more at Negri Electronics (http://negrielectronics.com/catalogsearch/result/?q=samsung+galaxy+s4+mini+dual+sim), $520, but they have the White Frost version as well. Overseas Electronics (http://welectronics.com/gsm/Samsung/SAMSUNG-I9192-GALAXY-S4-MINI-DUAL-SIM.HTML) have priced the Galaxy S4 mini dual-SIM at $580.
In India, Flipkart (http://www.flipkart.com/samsung-galaxy-s4-mini-i9192/p/itmdmmp3ra9wgzvt?pid=MOBDMMPYRYSGT7WA&otracker=from-search&srno=t_1&query=samsung+galaxy+s4+mini+dual+sim&ref=864ab7a8-33cb-47ba-be5a-5381fe003fd7) offers both color versions at INR27,990 ($470). In Egypt, Raya Shop (http://www.rayashop.com/Product/13903--Galaxy_S4_Mini_Dual_SIM#.UekYFz9rG70) carries it for EGP4,000 ($570).
It seems that Movistar (http://www.moviles.com/samsung/galaxy-s4-mini-dual-sim) in Spain will offer the Galaxy S4 mini dual-SIM on contract at