The seminal work [7] by Brezis and Merle showed that the bubbling solutions of the mean field equation have the property of mass concentration. Recently, Lin and Tarantello in [31] found that the "bubbling implies mass concentration" phenomena might not hold if there is a collapse of singularities. Furthermore, a sharp estimate [23] for the bubbling solutions has been obtained. In this paper, we prove that there exists at most one sequence of bubbling solutions if the collapsing singularity occurs. The main difficulty comes from that after re-scaling, the difference of two solutions locally converges to an element in the kernel space of the linearized operator. It is well-known that the kernel space is three dimensional. So the main technical ingredient of the proof is to show that the limit after re-scaling is orthogonal to the kernel space.Date: September 28, 2018. 1 2 YOUNGAE LEE AND CHANG-SHOU LIN However, there is a big difference when the collapsing singularities are considered. First, Lin and Tarantello in [31] observed a new phenomena such that blow-up solutions with collapsing singularities might not have the "concentration" property of its mass distribution. The general version was studied in [23]. To describe the results, let us consider the following equation:where lim t→0 q i (t) = q / ∈ {q d+1 , · · · , q N }, ∀ i = 1, · · · , d and q i (t) = q j (t) for i = j ∈ {1, · · · , d}. Then the following holds:t be a sequence of blow up solutions of (1.3) with ρ / ∈ 8πN. Then u * t blows up only at q ∈ M. Furthermore, there exists a function w * such that. So Theorem B tells us that the mass concentration does no longer hold if the collapsing singularity occurs. Indeed, we have lim t→0 M h * e u * t dv g < +∞, which is different from the situation described in Theorem A. We note that Theorem B could be improved provided that the following nondegeneracy condition holds: Definition 1.1. A solution w * of (1.4) is said non-degenerate, if the linearized problem