The ship lines cannot use 10' containers on boats. To get around this, they created "duocons." A duocon is basically two 10' containers welded together to make a 20'. It has doors on both ends and two extra pieces of corrugation metal tacked to the inside.
When you buy a duocon, you will need to cut it apart at the seam and reassemble the two 10' containers using the extra corrugation provided. This means you will need to know how to weld.
Duocons only come in "one trip" or like-new quality.