The usual formulation of the Plancherel theorem one writes $f(1)$ as an integral over the dual $\widehat G$. The support of the measure is the set of representations which weakly occur in $L^2(G)$. But this theorem doesn't give any multiplicities. For instance, let $D$ be a discrete series representation of $G=SL\_2({\mathbb R})$, then the $G\times G$ representation $D\otimes D^\*$ is a subrepresentation of $L^2(G)$.
What is its multiplicity? So what is the dimension of the space
$$
\mathrm{Hom}\_{G\times G}(D\otimes D^\*,L^2(G))?
$$
Likewise, what are the multiplicities of the continuous Hilbert integrals corresponding to the rest of the spectrum. I am looking for a paper which gives the $G\times G$ decomposition of $L^2(G)$.



Multiplicities in Plancherel theorem for SL2(R)
These multiplicities are all 1. Abstract Plancherel theorem (in e.g. Dixmier’s C\*-algebras, 18.8.1):


$L^2(G)=\int\_{\hat G}^\oplus H\_\pi\otimes H\_\pi^\*\,d\mu(\pi)$, for Plancherel measure $\mu$ on any type I loc. compact unimodular $G$.


