diff --git a/lib/matplotlib/tests/test_ticker.py b/lib/matplotlib/tests/test_ticker.py
index 12eafba9ea2b..fcfdf8ee92f5 100644
--- a/lib/matplotlib/tests/test_ticker.py
+++ b/lib/matplotlib/tests/test_ticker.py
@@ -1796,3 +1796,24 @@ def test_set_offset_string(formatter):
     assert formatter.get_offset() == ''
     formatter.set_offset_string('mpl')
     assert formatter.get_offset() == 'mpl'
+
+
+def test_minorticks_on_multi_fig():
+    """
+    Turning on minor gridlines in a multi-Axes Figure
+    that contains more than one boxplot and shares the x-axis
+    should not raise an exception.
+    """
+    fig, ax = plt.subplots()
+
+    ax.boxplot(np.arange(10), positions=[0])
+    ax.boxplot(np.arange(10), positions=[0])
+    ax.boxplot(np.arange(10), positions=[1])
+
+    ax.grid(which="major")
+    ax.grid(which="minor")
+    ax.minorticks_on()
+    fig.draw_without_rendering()
+
+    assert ax.get_xgridlines()
+    assert isinstance(ax.xaxis.get_minor_locator(), mpl.ticker.AutoMinorLocator)
diff --git a/lib/matplotlib/ticker.py b/lib/matplotlib/ticker.py
index 10ac8cad6ebc..1e9f0892c322 100644
--- a/lib/matplotlib/ticker.py
+++ b/lib/matplotlib/ticker.py
@@ -2903,7 +2903,7 @@ def __call__(self):
             _api.warn_external('AutoMinorLocator does not work on logarithmic scales')
             return []
 
-        majorlocs = self.axis.get_majorticklocs()
+        majorlocs = np.unique(self.axis.get_majorticklocs())
         if len(majorlocs) < 2:
             # Need at least two major ticks to find minor tick locations.
             # TODO: Figure out a way to still be able to display minor ticks with less