Consider the compact Lie groups $U(l)$ (the unitary group) and $U(1) \times SU(l)$ for some natural number $l$. Both the groups have the same Lie algebra $\frak{gl}_l$. Which means that they both have the "same" dense Peter-Weyl subalgebra $PW$ of their $C^*$-algebra of continuous functions.
The Gelfand-Naimark theorem says that the $C^*$-algebras of $U(l)$ and $U(1) \times SU(l)$ are non-isomorphic.
How to resolve the fact that both $C^*$-algebras are completions of the same algebra PW? Is it the case that both spaces have different norms, and or algebra structure, on PW? I guess this must be what is happening?