We present spectroscopic follow-up observations of Lyman Break Galaxies ( LBGs ) selected in the field surrounding the radio galaxy MRC 0316-257 at z \sim 3.13 ( 0316 ) .
Robust spectroscopic redshifts are determined for 20 out of 24 objects .
Three of the spectroscopically confirmed galaxies have 3.12 < z < 3.13 indicating that these objects reside in a protocluster structure previously found around the radio galaxy .
An additional 5 objects are found 1600 km s ^ { -1 } blue-shifted with respect to the main protocluster structure .
This is in addition to three [ O iii ] emitters found at this redshift in a previous study .
This is further evidence that a structure exists directly in front of the 0316 protocluster .
We estimate that the foreground structure is responsible for half of the surface overdensity of LBGs found in the field as a whole .
The foreground structure is associated with a strong surface density peak 1.4 Mpc to the North-West of the radio galaxy and a 2D Kolmogorov-Smirnov test indicates that the spatial distributions of the 0316 and foreground galaxies differ at the 3 \sigma level .
In addition , we compare the properties of protocluster , foreground structure and field galaxies , but we find no significant differences .
In terms of the nature of the two structures , a merger scenario is a possible option .
Simple merger dynamics indicates that the observed relative velocity of 1600 km s ^ { -1 } can be reproduced if the two structures have masses of \sim 5 \times 10 ^ { 14 } M _ { \odot } and have starting separations of around 2.5 to 3 Mpc .
It is also possible that the foreground structure is unrelated to the 0316 protocluster in which case the two structures will not interact before z = 0 .