A criterion for boundedness of composition operators acting on a class of Hilbert spaces of entire Dirichlet series, namely the class ℋ(E,βS)\documentclass[12pt]{minimal}
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\begin{document}$\mathcal {H}(E, \beta _{S})$\end{document}, was obtained in Hou et al. (J. Math. Anal. Appl. 401: 416–429, 2013) for those spaces that do not contain non-zero constant functions, while other possibilities were not studied. In this paper, we first provide a complete characterization of boundedness of composition operators on any space ℋ(E,βS)\documentclass[12pt]{minimal}
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\begin{document}$\mathcal {H}(E, \beta _{S})$\end{document}, which may or may not contain constant functions. We then study complex symmetry of composition operators on ℋ(E,βS)\documentclass[12pt]{minimal}
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\begin{document}$\mathcal {H}(E, \beta _{S})$\end{document}, via analysis of composition conjugations.