Add barebones analyser definition and failing tests

LocalisationAnalyser.Tests/Analysers/LocalisationKeyUsedMultipleTimesInClassAnalyserTests.cs

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+using System.Threading.Tasks;
+using LocalisationAnalyser.Analysers;
+using Xunit;
+
+namespace LocalisationAnalyser.Tests.Analysers
+{
+    public class LocalisationKeyUsedMultipleTimesInClassAnalyserTests : AbstractAnalyserTests<LocalisationKeyUsedMultipleTimesInClassAnalyser>
+    {
+        [Theory]
+        [InlineData("DuplicatedLocalisationKeys")]
+        public Task RunTest(string name) => Check(name);
+    }
+}

LocalisationAnalyser.Tests/Resources/Analysers/DuplicatedLocalisationKeys/Localisation/CommonStrings.txt

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+using osu.Framework.Localisation;
+
+namespace TestProject.Localisation
+{
+    public static class CommonStrings
+    {
+        private const string prefix = @"TestProject.Localisation.Common";
+
+        /// <summary>
+        /// "first string"
+        /// </summary>
+        public static LocalisableString FirstString => new TranslatableString(getKey([|@"first_string"|]), @"first string");
+
+        /// <summary>
+        /// "second string"
+        /// </summary>
+        public static LocalisableString SecondString => new TranslatableString(getKey([|@"first_string"|]), @"second string");
+
+        /// <summary>
+        /// "third string"
+        /// </summary>
+        public static LocalisableString ThirdString => new TranslatableString(getKey(@"third_string"), @"third string");
+
+        /// <summary>
+        /// "first string with arguments (like {0})"
+        /// </summary>
+        public static LocalisableString FirstStringWithArguments(string test) => new TranslatableString(getKey([|@"first_string"|]), @"first string with arguments (like {0})");
+
+        /// <summary>
+        /// "second string with arguments (like {0})"
+        /// </summary>
+        public static LocalisableString SecondStringWithArguments(string test) => new TranslatableString(getKey(@"second_string_with_args"), @"second string with arguments (like {0})");
+
+        private static string getKey(string key) => $@"{prefix}:{key}";
+    }
+}

LocalisationAnalyser/Analysers/LocalisationKeyUsedMultipleTimesInClassAnalyser.cs

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+using System.Collections.Immutable;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.Diagnostics;
+
+namespace LocalisationAnalyser.Analysers
+{
+    [DiagnosticAnalyzer(LanguageNames.CSharp)]
+    public class LocalisationKeyUsedMultipleTimesInClassAnalyser : AbstractMemberAnalyser
+    {
+        public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics =>
+            ImmutableArray.Create(DiagnosticRules.LOCALISATION_KEY_USED_MULTIPLE_TIMES_IN_CLASS);
+    }
+}