Meso structures:

The molecule in the below diagram has two identical asymmetric centers but is not chiral. This structure's mirror images are superimposable and thus the compound cannot be chiral. 

Figure:   (a) Meso structure showing asymmetric centers; (b) plane of symmetry in meso structure.

Figure:   Stereoisomers of threonine.

This  is  since  the  molecule  consists of  a  plane  of  symmetry  as illustrated in the diagram in which the molecule has been rotated around the central C-C bond. A structure like this is called a meso structure.