Fixed #7339, removed doubled axis ticks in case of too dense ticks.

js/parts/Axis.js

@@ -2299,6 +2299,10 @@ H.extend(Axis.prototype, /** @lends Highcharts.Axis.prototype */{
 			// Too dense ticks, keep only the first and last (#4477)
 			if (tickPositions.length > this.len) {
 				tickPositions = [tickPositions[0], tickPositions.pop()];
+				// Reduce doubled value (#7339)
+				if (tickPositions[0] === tickPositions[1]) {
+					tickPositions.length = 1;
+				}
 			}
 
 			this.tickPositions = tickPositions;

js/parts/Chart.js

@@ -1708,7 +1708,9 @@ extend(Chart.prototype, /** @lends Highcharts.Chart.prototype */ {
 
 		// Record preliminary dimensions for later comparison
 		tempWidth = chart.plotWidth;
-		tempHeight = chart.plotHeight = chart.plotHeight - 21; // 21 is the most common correction for X axis labels
+		// 21 is the most common correction for X axis labels
+		// use Math.max to prevent negative plotHeight
+		tempHeight = chart.plotHeight = Math.max(chart.plotHeight - 21, 0);
 
 		// Get margins by pre-rendering axes
 		each(axes, function (axis) {

samples/unit-tests/axis/ticks/demo.js

@@ -27,6 +27,37 @@ QUnit.test('Ticks for a single point.', function (assert) {
         -0.3,
         'single tick and increased extremes for a single point'
     );
+
+    // Must be on init - redraw was fixing the issue
+    chart = Highcharts.chart('container', {
+        series: [{
+            type: 'bar',
+            data: [10]
+        }],
+        chart: {
+            height: 30,
+            inverted: true,
+            spacing: [6, 10, 6, 10]
+        },
+        legend: {
+            enabled: false
+        },
+        title: {
+            text: ''
+        },
+        yAxis: [{
+            visible: false
+        }],
+        xAxis: [{
+            visible: false
+        }]
+    });
+
+    assert.strictEqual(
+        chart.xAxis[0].tickPositions.length,
+        1,
+        'no doulbed tick for a small plot height (#7339)'
+    );
 });
 
 QUnit.test(