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We investigate the collision dynamics of discrete soliton (DS) pair in a realistic semi-infinite nonlinear waveguide array (WA) in the context of \textit{diffractive resonant radiation} (DifRR). Depending on the initial amplitude ($A_0$) and wavenumber ($k_0$), a co-moving pair of identical DSs either collide elastically or merge to form a discrete \textit{breather}. For large amplitude and small wavenumber, DSs form a bound state and do not interact . We map the domain of interaction by iterative simulation and present a phase plot in ($A_0$-$k_0$) parameter space. A variational technique is developed where the interaction term is considered as perturbation. A proper choice of Lagrangian density and the \textit{ansätz} function followed by the Ritz optimization leads us to a set of ordinary differential equations that describe the collision mechanism. The analytical result corroborate well with numerical data. We further investigate the role of initial phase detuning between two identical DS and observe a periodic energy exchange during their interaction. Varied degree of energy exchange occurs when two DS with different wavenumbers collide which results in three distinct output states accompanied by DifRR generation. Extending our investigation to a more generalized condition by taking different amplitudes and wavenumber of DS pair, we find an unique secondary radiation in $k$-space which is originated due to the collision of solitons. The nature of this collision mediated secondary radiation is found to be different from usual DifRR. Our results shed light on the interesting aspects of the collision dynamics of DS pair in nonlinear WA and useful in understanding the complex mechanism.