  Let $A$ be a subset of the cyclic group $\mathbf{Z}/p\mathbf{Z}$ with $p$
prime. It is a well-studied problem to determine how small $|A|$ can be if
there is no unique sum in $A+A$, meaning that for every two elements
$a_1,a_2\in A$, there exist $a_1',a_2'\in A$ such that $a_1+a_2=a_1'+a_2'$ and
$\{a_1,a_2\}\neq \{a_1',a_2'\}$. Let $m(p)$ be the size of a smallest subset of
$\mathbf{Z}/p\mathbf{Z}$ with no unique sum. The previous best known bounds are
$\log p \ll m(p)\ll \sqrt{p}$. In this paper we improve both the upper and
lower bounds to $\omega(p)\log p \leqslant m(p)\ll (\log p)^2$ for some
function $\omega(p)$ which tends to infinity as $p\to \infty$. In particular,
this shows that for any $B\subset \mathbf{Z}/p\mathbf{Z}$ of size
$|B|<\omega(p)\log p$, its sumset $B+B$ contains a unique sum. We also obtain
corresponding bounds on the size of the smallest subset of a general Abelian
group having no unique sum.
