[hist] disallow equal bin edges

Fixes #20185

credit goes to VanyaBelyaev

Author: ferdymercury

hist/hist/src/TAxis.cxx

@@ -800,7 +800,7 @@ void TAxis::Set(Int_t nbins, const Float_t *xbins)
    for (bin=0; bin<= fNbins; bin++)
       fXbins.fArray[bin] = xbins[bin];
    for (bin=1; bin<= fNbins; bin++)
-      if (fXbins.fArray[bin] < fXbins.fArray[bin-1])
+      if (fXbins.fArray[bin] <= fXbins.fArray[bin - 1])
          Error("TAxis::Set", "bins must be in increasing order");
    fXmin      = fXbins.fArray[0];
    fXmax      = fXbins.fArray[fNbins];
@@ -818,7 +818,7 @@ void TAxis::Set(Int_t nbins, const Double_t *xbins)
    for (bin=0; bin<= fNbins; bin++)
       fXbins.fArray[bin] = xbins[bin];
    for (bin=1; bin<= fNbins; bin++)
-      if (fXbins.fArray[bin] < fXbins.fArray[bin-1])
+      if (fXbins.fArray[bin] <= fXbins.fArray[bin - 1])
          Error("TAxis::Set", "bins must be in increasing order");
    fXmin      = fXbins.fArray[0];
    fXmax      = fXbins.fArray[fNbins];

hist/hist/test/test_TH1.cxx

@@ -11,6 +11,8 @@
 #include <random>
 #include <vector>
 
+#include "ROOT/TestSupport.hxx"
+
 // StatOverflows TH1
 TEST(TH1, StatOverflows)
 {
@@ -312,3 +314,9 @@ TEST(TH1, SetBufferedSumw2)
    EXPECT_FLOAT_EQ(h1.GetBinContent(1), Entries * Weight);
    EXPECT_FLOAT_EQ(h1.GetBinError(1), std::sqrt(Entries * Weight * Weight));
 }
+
+// https://github.com/root-project/root/issues/20185
+TEST(TAxis, EqualBinEdges)
+{
+   ROOT_EXPECT_ERROR(TAxis _({1, 1}), "TAxis::Set", "bins must be in increasing order");
+}