Lemma 12.17.2. Let $\mathcal{A}$ be an additive monoidal category. If $Y_ i$, $i = 1, 2$ are left duals of $X_ i$, $i = 1, 2$, then $Y_1 \oplus Y_2$ is a left dual of $X_1 \oplus X_2$.
Proof. Follows from uniqueness of adjoints and Categories, Remark 4.43.7. $\square$
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