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We report experimental and three-dimensional discrete dislocation dynamics measurements to characterize the effect of high starting dislocation density on size-affected flow. Microcrystals were focused ion beam milled into a pre-strained bulk Ni single crystal to obtain a dense heterogeneous dislocation cell structure. The strength-scaling exponent is shown to decreases considerably with increasing dislocation density. Cutting the pre-existing forest during microcrystal fabrication can lead to alternate pathways for the advancing slipped areas to defeat the forest, resulting in a lower strength than the pre-strained bulk crystal.