Exercise tracking and Fitbit logging

When you manually log that workout on MFP, it syncs back to Fitbit account and REPLACES whatever Fitbit had for that chunk of time. Fitbit is a replace-only system, the last added is what counts if any conflicts of time.

So no doubling up - but you are likely replacing a better Fitbit device estimate with the MFP database entry.
Tap and hold on that Adjustment line - you'll see that the math removes any workouts MFP knows about so there is no doubling up.

Now, when you log manually on Fitbit, it uses the same database MFP uses, so no big whoop there, like if you needed a better estimate of workout calories burned than HR would provide - like Weights, or Swimming perhaps.
But for walking, I'm guessing you already started a workout on the device, so there is already an activity record there, with HR, steps, distance, calorie burn.

Now Fitbit uses their database better for calorie burn when you do need to manually log it, so better to manually log there. But for walking just start a workout on Fitbit.

Plus Fitbit has a much better Fitness diary compared to MFP, and graphs and progress.
And when you log on MFP it requires another sync to Fitbit of that workout, and then another sync of the daily burn back to MFP. With the Fitbit syncing issues you are just increasing your odds of having an issue.

Stop manually logging the walking, the calorie burn is already contained in the Fitbit daily calorie burn that is sent to MFP that causes the Adjustment line, along with extra activity you may have done, or perhaps less activity than MFP was expecting.
All taken care of in that Adjustment.
You delete the workout now, that adjustment should go up since MFP won't be dealing with it.

If you need your friends list to see you did a workout, just make a wall post about it, with more interesting info than a simple 1-liner gives.

https://www.myfitnesspal.com/apps

Find Fitbit in list (similar location on MFP App) and click only button available and go through process as instructed.