  A combination of searches for a new resonance decaying into a Higgs boson
pair is presented, using up to 139 fb$^{-1}$ of $pp$ collision data at
$\sqrt{s}=13$ TeV recorded with the ATLAS detector at the LHC. The combination
includes searches performed in three decay channels: $b\bar{b}b\bar{b}$,
$b\bar{b}\tau^+\tau^-$ and $b\bar{b}\gamma\gamma$. No excess above the expected
Standard Model background is observed and upper limits are set at the 95%
confidence level on the production cross section of Higgs boson pairs
originating from the decay of a narrow scalar resonance with mass in the range
251 GeV-5 TeV. The observed (expected) limits are in the range 0.96-600 fb
(1.2-390 fb). The limits are interpreted in the Type-I Two-Higgs-Doublet Model
and the Minimimal Supersymmetric Standard Model, and constrain parameter space
not previously excluded by other searches.
