diff --git a/CHANGELOG.md b/CHANGELOG.md
index 91dc8a1121..3cd8252937 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@ All notable changes to this project will be documented in this file.
 - [Multiple] Show DLC in recommendations list (#3038 by: HebaruSan; reviewed: DasSkelett)
 - [Multiple] Prompt user to overwrite manually installed files (#3043 by: HebaruSan; reviewed: DasSkelett)
 - [GUI] Master search bar and misc GUI clean-up (#3041 by: HebaruSan; reviewed: DasSkelett)
+- [Core] Don't try to install multiple versions of the same mod (#3051 by: HebaruSan; reviewed: DasSkelett)
 
 ### Bugfixes
 
diff --git a/Core/Relationships/RelationshipResolver.cs b/Core/Relationships/RelationshipResolver.cs
index ed66724b32..04cdb6e851 100644
--- a/Core/Relationships/RelationshipResolver.cs
+++ b/Core/Relationships/RelationshipResolver.cs
@@ -410,7 +410,9 @@ private void ResolveStanza(IEnumerable<RelationshipDescriptor> stanza, Selection
                 var descriptor1 = descriptor;
                 List<CkanModule> candidates = descriptor
                     .LatestAvailableWithProvides(registry, kspversion, modlist.Values)
-                    .Where(mod => descriptor1.WithinBounds(mod) && MightBeInstallable(mod))
+                    .Where(mod => !modlist.ContainsKey(mod.identifier)
+                        && descriptor1.WithinBounds(mod)
+                        && MightBeInstallable(mod))
                     .ToList();
                 if (candidates.Count == 0)
                 {
@@ -418,7 +420,9 @@ private void ResolveStanza(IEnumerable<RelationshipDescriptor> stanza, Selection
                     // (conflicts will still be caught below)
                     candidates = descriptor
                         .LatestAvailableWithProvides(registry, kspversion)
-                        .Where(mod => descriptor1.WithinBounds(mod) && MightBeInstallable(mod))
+                        .Where(mod => !modlist.ContainsKey(mod.identifier)
+                            && descriptor1.WithinBounds(mod)
+                            && MightBeInstallable(mod))
                         .ToList();
                 }
 
@@ -519,7 +523,7 @@ private void Add(CkanModule module, SelectionReason reason)
             {
                 // We should never be adding something twice!
                 log.ErrorFormat("Assertion failed: Adding {0} twice in relationship resolution", module.identifier);
-                throw new ArgumentException("Already contains module:" + module.identifier);
+                throw new ArgumentException("Already contains module: " + module.identifier);
             }
             modlist.Add(module.identifier, module);
             if (!reasons.ContainsKey(module))