diff --git a/test/api/unit/models/challenge.test.js b/test/api/unit/models/challenge.test.js
index 5c69ef4c27..d07cf9e822 100644
--- a/test/api/unit/models/challenge.test.js
+++ b/test/api/unit/models/challenge.test.js
@@ -35,6 +35,33 @@ describe('Challenge Model', () => {
       notes: 'test notes',
     },
   };
+  const tasks2ToTest = {
+    habit: {
+      text: 'test habit 2',
+      type: 'habit',
+      up: false,
+      down: true,
+      notes: 'test notes',
+    },
+    todo: {
+      text: 'test todo 2',
+      type: 'todo',
+      notes: 'test notes',
+    },
+    daily: {
+      text: 'test daily 2',
+      type: 'daily',
+      frequency: 'daily',
+      everyX: 5,
+      startDate: new Date(),
+      notes: 'test notes',
+    },
+    reward: {
+      text: 'test reward 2',
+      type: 'reward',
+      notes: 'test notes',
+    },
+  };
 
   beforeEach(async () => {
     guild = new Group({
@@ -146,6 +173,60 @@ describe('Challenge Model', () => {
         expect(syncedTask.attribute).to.eql('str');
       });
 
+      it('should add challenge tag back to user upon syncing challenge tasks to a user with challenge tag removed', async () => {
+        await challenge.addTasks([task]);
+
+        const newMember = new User({
+          guilds: [guild._id],
+        });
+        await newMember.save();
+        await challenge.syncTasksToUser(newMember);
+
+        let updatedNewMember = await User.findById(newMember._id).exec();
+        const updatedNewMemberId = updatedNewMember._id;
+
+        updatedNewMember.tags = [];
+        await updatedNewMember.save();
+
+        const taskValue2 = tasks2ToTest[taskType];
+        const task2 = new Tasks[`${taskType}`](Tasks.Task.sanitize(taskValue2));
+        task2.challenge.id = challenge._id;
+        await challenge.addTasks([task2]);
+        await challenge.syncTasksToUser(updatedNewMember);
+
+        updatedNewMember = await User.findById(updatedNewMemberId).exec();
+
+        expect(updatedNewMember.tags.length).to.equal(1);
+        expect(updatedNewMember.tags[0].id).to.equal(challenge._id);
+        expect(updatedNewMember.tags[0].name).to.equal(challenge.shortName);
+      });
+
+      it('should not add a duplicate challenge tag to user upon syncing challenge tasks to a user with existing challenge tag', async () => {
+        await challenge.addTasks([task]);
+
+        const newMember = new User({
+          guilds: [guild._id],
+        });
+        await newMember.save();
+        await challenge.syncTasksToUser(newMember);
+
+        let updatedNewMember = await User.findById(newMember._id).exec();
+        const updatedNewMemberId = updatedNewMember._id;
+
+        const taskValue2 = tasks2ToTest[taskType];
+        const task2 = new Tasks[`${taskType}`](Tasks.Task.sanitize(taskValue2));
+        task2.challenge.id = challenge._id;
+        await challenge.addTasks([task2]);
+        await challenge.syncTasksToUser(updatedNewMember);
+
+        updatedNewMember = await User.findById(updatedNewMemberId);
+
+        expect(updatedNewMember.tags.length).to.equal(8);
+        expect(updatedNewMember.tags[7].id).to.equal(challenge._id);
+        expect(updatedNewMember.tags[7].name).to.equal(challenge.shortName);
+        expect(updatedNewMember.tags.filter(tag => tag.id === challenge._id).length).to.equal(1);
+      });
+
       it('syncs challenge tasks to a user with the existing task', async () => {
         await challenge.addTasks([task]);
 
diff --git a/website/server/models/challenge.js b/website/server/models/challenge.js
index e550209884..1643c52109 100644
--- a/website/server/models/challenge.js
+++ b/website/server/models/challenge.js
@@ -234,8 +234,21 @@ async function _addTaskFn (challenge, tasks, memberId) {
     }));
   });
 
-  // Update the user
-  toSave.unshift(User.update({ _id: memberId }, updateTasksOrderQ).exec());
+  // Update the tag list of the user document of a participating member of the challenge
+  // such that a tag representing the challenge into which the task to be added will
+  // be added to the user tag list if and only if the tag does not exist already.
+  const addToChallengeTagSet = {
+    $addToSet: {
+      tags: {
+        id: challenge._id,
+        name: challenge.shortName,
+        challenge: true,
+      },
+    },
+  };
+  const updateUserParams = { ...updateTasksOrderQ, ...addToChallengeTagSet };
+  toSave.unshift(User.update({ _id: memberId }, updateUserParams).exec());
+
   return Promise.all(toSave);
 }
 
@@ -244,7 +257,7 @@ schema.methods.addTasks = async function challengeAddTasks (tasks) {
   const challenge = this;
   const membersIds = await _fetchMembersIds(challenge._id);
 
-  const queue = new TaskQueue(Promise, 25); // process only 5 users concurrently
+  const queue = new TaskQueue(Promise, 25); // process only this many users concurrently
 
   await Promise.all(membersIds.map(queue.wrap(memberId => _addTaskFn(challenge, tasks, memberId))));
 };