Consider a closed $O^*$--algebra $\mathcal{M}$ on a dense linear subspace $\mathcal{D}$ of a Hilbert space $\mathcal{H}$, a locally compact group $G$ with left invariant Haar measure $ds$ and an action $\alpha$ of $G$ on $\mathcal{M}$. Under some natural conditions, the $O^*$--crossed product $\mathcal{M}\underset{\alpha}{\overset{O^*}{\rtimes}}G$ of $\mathcal{M}$ and the $GW^*$--crossed product $\mathcal{M}\underset{\alpha}{\overset{GW^*}{\rtimes}}G$ are introduced. When $G$ is also abelian, the dual action $\widehat{\alpha}$ of the dual group $\widehat{G}$ on $\mathcal{M}\underset{\alpha}{\overset{O^*}{\rtimes}}G$ and on $\mathcal{M}\underset{\alpha}{\overset{GW^*}{\rtimes}}G$ is defined, which makes it possible to study the crossed products $(\mathcal{M}\underset{\alpha}{\overset{O^*}{\rtimes}}G)\underset{\widehat{\alpha}}{\overset{O^*}{\rtimes}}\widehat{G}$ and $\mathcal{M}\underset{\alpha}{\overset{GW^*}{\rtimes}}G)\underset{\widehat{\alpha}}{\overset{GW^*}{\rtimes}}\widehat{G}$. In case of modular actions, these constructions are used to obtain results on duality of type $\mathrm{II}$--like and type $\mathrm{III}$--like $GW^*$--algebras.