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We study the relation between optical duobinary modulation and spectral efficiency in wavelength-division-multiplexed systems. We show that in spite of the fact that duobinary modulation reduces the optical bandwidth of near-rectangular signals, it offers no advantages over binary modulation in terms of the average high-frequency spectral content. This is true as long as the same basic pulse shapes are used in both cases. An improvement in the spectral efficiency can only be achieved by using narrow-band pulses in the duobinary case. This is enabled by the fact that the duobinary code is more robust to narrow-band filtering.