Samsung China has already made official the dual-SIM variant of the Samsung Galaxy S4, carrying the model number GT-I9502.
The dual-SIM version of the Galaxy S4 will have specs similar to the I9500 model meaning it will sport an Exynos 5 Octa chipset. The rest of the hardware are the same except for the extra slot allocated for your secondary SIM.
The Samsung I9502 Galaxy S4 is only available for the Chinese market as of the moment. No confirmation yet if Samsung will make it available in other markets as well.