diff --git a/src/extension.ts b/src/extension.ts
index c8436a8..ab87963 100644
--- a/src/extension.ts
+++ b/src/extension.ts
@@ -6,6 +6,7 @@ import * as crypto from 'crypto';
 import { documentationLinkMap, getPremiumCertLink } from './util/documentation';
 import { runCommand } from './util/scripts';
 import { looksLikePath, resolvePath, findWorkspaceRoot } from './util/path';
+import { diagnosticsUnion } from './util/diagnostics';
 
 // To keep track of document changes we save hashed versions of their content to this record
 let documentHashMemory : Record<string, string> = {};
@@ -349,7 +350,6 @@ async function runCppcheckOnFileXML(
 
             const errors = result.results?.errors?.[0]?.error || [];
             const diagnostics: Record<string, vscode.Diagnostic[]> = {};
-
             for (const e of errors) {
                 const isCriticalError = criticalWarningTypes.includes(e.$.id);
                 const locations = e.location || [];
@@ -358,12 +358,13 @@ async function runCppcheckOnFileXML(
                 }
 
                 const mainLoc = locations[locations.length - 1].$;
-                
                 // If main location is not current file, we are not using a project file and warning is not critical then skip displaying warning
                 if (!isCriticalError && usingProjectFile && !filePath.endsWith(mainLoc.file)) {
                     continue;
                 }
 
+                const mainLocDocument = await vscode.workspace.openTextDocument(mainLoc.file);
+
                 // Cppcheck line number is 1-indexed, while VS Code uses 0-indexing
                 let line = Number(mainLoc.line) - 1;
                 // Invalid line number usually means non-analysis output 
@@ -377,7 +378,7 @@ async function runCppcheckOnFileXML(
 
                 // Cppcheck col number is 1-indexed, while VS Code uses 0-indexing
                 let col = Number(mainLoc.column) - 1;
-                if (isNaN(col) || col < 0 || col > document.lineAt(line).text.length) {
+                if (isNaN(col) || col < 0 || col > mainLocDocument.lineAt(line).text.length) {
                     col = 0;
                 }
 
@@ -386,7 +387,7 @@ async function runCppcheckOnFileXML(
                     continue;
                 }
 
-                const range = new vscode.Range(line, col, line, document.lineAt(line).text.length);
+                const range = new vscode.Range(line, col, line, mainLocDocument.lineAt(line).text.length);
                 const diagnostic = new vscode.Diagnostic(range, e.$.msg, severity);
                 diagnostic.source = "cppcheck";
                 // If we have a link to documentation, include it
@@ -411,11 +412,11 @@ async function runCppcheckOnFileXML(
                         continue;
                     }
 
+                    const relatedDocument = await vscode.workspace.openTextDocument(loc.file);
                     const relatedRange = new vscode.Range(
                         lLine, lCol,
-                        lLine, document.lineAt(lLine).text.length
+                        lLine, relatedDocument.lineAt(lLine).text.length
                     );
-                    const relatedDocument = await vscode.workspace.openTextDocument(loc.file);
                     relatedInfos.push(
                         new vscode.DiagnosticRelatedInformation(
                             new vscode.Location(relatedDocument?.uri ?? '', relatedRange),
@@ -423,6 +424,7 @@ async function runCppcheckOnFileXML(
                         )
                     );
                 }
+
                 if (relatedInfos.length > 0) {
                     diagnostic.relatedInformation = relatedInfos;
                 }
@@ -444,7 +446,13 @@ async function runCppcheckOnFileXML(
             }
             const sourceDocumentUri = document.uri.toString();
             for (const uri of Object.keys(diagnostics)) {
-                diagnosticCollection.set(vscode.Uri.parse(uri), diagnostics[uri]);
+                var newDiagnostics = diagnostics[uri];
+                // If file has existing diagnostics from analyzing other files we do not want to overwrite those
+                const existingDiagnostics = diagnosticCollection.get(vscode.Uri.parse(uri));
+                if (existingDiagnostics) {
+                    newDiagnostics = diagnosticsUnion(newDiagnostics, existingDiagnostics.flat());
+                }
+                diagnosticCollection.set(vscode.Uri.parse(uri), newDiagnostics);
                 if (fileRelationMap[uri] === null ||fileRelationMap[uri] === undefined) {
                     fileRelationMap[uri] = new Set;
                 }
diff --git a/src/util/diagnostics.ts b/src/util/diagnostics.ts
new file mode 100644
index 0000000..0054b1a
--- /dev/null
+++ b/src/util/diagnostics.ts
@@ -0,0 +1,23 @@
+import * as vscode from 'vscode';
+
+export function diagnosticsUnion(diagnosticsA : vscode.Diagnostic[], diagnosticB : vscode.Diagnostic[]) : vscode.Diagnostic[] {
+    const diagnosticsUnion = new Array<vscode.Diagnostic>;
+    // Add all elements from diagnosticsA to result array
+    diagnosticsUnion.push(...diagnosticsA);
+
+    // Add all elements present in diagnosticsB but not in diagnosticsA to result array 
+    for (const diagnostic of diagnosticB) {
+        if (!diagnosticsA.some((d) => {
+            if (typeof(diagnostic?.code) === "object" && typeof(diagnostic?.code) !== null && typeof(d?.code) === "object" && typeof(d?.code) !== null) {
+                return diagnostic.code.value === d.code.value && diagnostic.range.isEqual(d.range);
+            } else {
+                return diagnostic.code === d.code && diagnostic.range.isEqual(d.range);
+            }
+        })) {
+            diagnosticsUnion.push(diagnostic);
+        }
+    }
+    
+    // Return result
+    return diagnosticsUnion;
+}
\ No newline at end of file