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We investigate the effect of including a significant ``binary twin''population (binaries with almost equal mass stars, q = M2/M1 > 0.95) for theproduction of double compact objects and some resulting consequences, includingLIGO inspiral rate and some properties of short-hard gamma-ray bursts. Weemploy very optimistic assumptions on the twin fraction (50%) among allbinaries, and therefore our calculations place an upper limits on the influenceof twins on double compact object populations. We show that for LIGO the effectof including twins is relatively minor: although the merger rates does indeedincrease when twins are considered, the rate increase is fairly small (1.5).Also, chirp mass distribution for double compact objects formed with or withouttwins are almost indistinguishable. If double compact object are short-hard GRBprogenitors, including twins in population synthesis calculations does notalter significantly the earlier rate predictions for the event rate. However,for one channel of binary evolution, introducing twins more than doubles therate of ``very prompt'' NS-NS mergers (time to merger less than 1 Myr) comparedto models with the ``flat'' q distribution. In that case, 70% of all NS-NSbinaries merge within 100 Myr after their formation, indicating a possibilityof a very significant population of ``prompt'' short-hard gamma-ray bursts,associated with star forming galaxies. We also point out that, independent ofassumptions, fraction of such prompt neutron star mergers is always high,35--70%. We note that recent observations (e.g., Berger et al.) indicate thatfraction of short-hard GRBs found in young hosts is at least 40% and possiblyeven 80%.

Short-Hard Gamma-Ray Bursts in Young Host Galaxies: the Effect of Prompt Twins