diff --git a/extensions/sql-database-projects/src/common/constants.ts b/extensions/sql-database-projects/src/common/constants.ts
index 5eb2685158..c1dc47a399 100644
--- a/extensions/sql-database-projects/src/common/constants.ts
+++ b/extensions/sql-database-projects/src/common/constants.ts
@@ -198,6 +198,8 @@ export function fileOrFolderDoesNotExist(name: string) { return localize('fileOr
 export function cannotResolvePath(path: string) { return localize('cannotResolvePath', "Cannot resolve path {0}", path); }
 export function fileAlreadyExists(filename: string) { return localize('fileAlreadyExists', "A file with the name '{0}' already exists on disk at this location. Please choose another name.", filename); }
 export function folderAlreadyExists(filename: string) { return localize('folderAlreadyExists', "A folder with the name '{0}' already exists on disk at this location. Please choose another name.", filename); }
+export function fileAlreadyAddedToProject(filepath: string) { return localize('fileAlreadyAddedToProject', "A file with the path '{0}' has already been added to the project", filepath); }
+export function folderAlreadyAddedToProject(folderpath: string) { return localize('folderAlreadyAddedToProject', "A folder with the path '{0}' has already been added to the project", folderpath); }
 export function invalidInput(input: string) { return localize('invalidInput', "Invalid input: {0}", input); }
 export function unableToCreatePublishConnection(input: string) { return localize('unableToCreatePublishConnection', "Unable to construct connection: {0}", input); }
 export function circularProjectReference(project1: string, project2: string) { return localize('cicularProjectReference', "Circular reference from project {0} to project {1}", project1, project2); }
diff --git a/extensions/sql-database-projects/src/models/project.ts b/extensions/sql-database-projects/src/models/project.ts
index ae815814cf..5f22318bc2 100644
--- a/extensions/sql-database-projects/src/models/project.ts
+++ b/extensions/sql-database-projects/src/models/project.ts
@@ -266,6 +266,11 @@ export class Project {
 	public async addFolderItem(relativeFolderPath: string): Promise<FileProjectEntry> {
 		const absoluteFolderPath = path.join(this.projectFolderPath, relativeFolderPath);
 
+		// check if folder already has been added to sqlproj
+		if (this.files.find(f => f.relativePath.toLowerCase() === relativeFolderPath.toLowerCase())) {
+			throw new Error(constants.folderAlreadyAddedToProject((relativeFolderPath)));
+		}
+
 		//If folder doesn't exist, create it
 		let exists = await utils.exists(absoluteFolderPath);
 		if (!exists) {
@@ -292,6 +297,11 @@ export class Project {
 			throw new Error(constants.fileAlreadyExists(path.parse(absoluteFilePath).name));
 		}
 
+		// check if file already has been added to sqlproj
+		if (this.files.find(f => f.relativePath.toLowerCase() === relativeFilePath.toLowerCase())) {
+			throw new Error(constants.fileAlreadyAddedToProject((relativeFilePath)));
+		}
+
 		// create the file if contents were passed in
 		if (contents) {
 			await fs.mkdir(path.dirname(absoluteFilePath), { recursive: true });
diff --git a/extensions/sql-database-projects/src/test/project.test.ts b/extensions/sql-database-projects/src/test/project.test.ts
index 0a4ad9fa72..aa63235054 100644
--- a/extensions/sql-database-projects/src/test/project.test.ts
+++ b/extensions/sql-database-projects/src/test/project.test.ts
@@ -13,7 +13,7 @@ import * as constants from '../common/constants';
 
 import { promises as fs } from 'fs';
 import { Project, EntryType, SystemDatabase, SystemDatabaseReferenceProjectEntry, SqlProjectReferenceProjectEntry } from '../models/project';
-import { exists, convertSlashesForSqlProj } from '../common/utils';
+import { exists, convertSlashesForSqlProj, trimChars, trimUri } from '../common/utils';
 import { Uri, window } from 'vscode';
 import { IDacpacReferenceSettings, IProjectReferenceSettings, ISystemDatabaseReferenceSettings } from '../models/IDatabaseReferenceSettings';
 
@@ -569,6 +569,27 @@ describe('Project: sqlproj content operations', function (): void {
 		should(newProject.noneDeployScripts.find(f => f.type === EntryType.File && f.relativePath === convertSlashesForSqlProj(postDeploymentScriptFilePath2))).not.equal(undefined, 'File Script.PostDeployment2.sql not read');
 
 	});
+
+	it('Should not allow adding duplicate file/folder entries in sqlproj', async function (): Promise<void> {
+		projFilePath = await testUtils.createTestSqlProjFile(baselines.newProjectFileBaseline);
+		const project: Project = await Project.openProject(projFilePath);
+		const fileList = await testUtils.createListOfFiles(path.dirname(projFilePath));
+
+		// verify first entry in list is a folder
+		const existingFolderUri = fileList[0];
+		const folderStats =  await fs.stat(existingFolderUri.fsPath);
+		should(folderStats.isDirectory()).equal(true, 'First entry in fileList should be a folder');
+		await project.addToProject([existingFolderUri]);
+		const folderRelativePath = trimChars(trimUri(Uri.file(projFilePath), existingFolderUri), '');
+		testUtils.shouldThrowSpecificError(async () => await project.addToProject([existingFolderUri]), constants.folderAlreadyAddedToProject(folderRelativePath));
+
+		// verify duplicate file can't be added
+		const existingFileUri = fileList[1];
+		const fileStats = await fs.stat(existingFileUri.fsPath);
+		should(fileStats.isFile()).equal(true, 'Second entry in fileList should be a file');
+		const fileRelativePath = trimChars(trimUri(Uri.file(projFilePath), existingFileUri), '/');
+		testUtils.shouldThrowSpecificError(async () => await project.addToProject([existingFileUri]), constants.fileAlreadyAddedToProject(fileRelativePath));
+	});
 });
 
 describe('Project: add SQLCMD Variables', function (): void {