One of the critical findings in recognition memory is the null list-strength effect (LSE), which states that repeating study items does not hurt the performance of other studied items. Episodic memory models were able to predict the null LSE by using the principle of differentiation, in which repetitions of an item accumulate into a single strong memory trace. A hypothesized boundary of differentiation is that repetitions of an item in different contexts will create new traces. Two experiments tested this hypothesis by repeating words across different study-test cycles rather than within a single list followed by a test on all of the studied lists. Results indicated that as the proportion of strong items increased, there was both a null LSE and a non-significant decrease in the FAR, which is contrary to the predicted strength-based mirror effect. These two results in tandem provide a challenge for differentiation models.