1 day duration - finish day is different from Start day

you may not realize it but Primavera uses hours even though your planning unit is in days. 

your times are incorrect and possibly your remaining durations are not in whole number of days especially if you are using duration % complete to calculate the remaking durations

here is the fix

go to edit users preferences

date tab

on bottom show minutes and check 12 hour

next go to your project directory highlight your project click on your Planned Start Date make sure that the TIME is 8:00 am or the earliest time setup in all of your calendars that you are using for that project.

Next click on your must finish by date and make sure that the TIME is 5:00 pm or the latest time setup in all of your calendars that you are using for that project.

Next click on the data date make sure that the TIME is 8:00 am or the earliest time setup in all of your calendars that you are using for that project.

now for any start milestone constraints make sure that the TIME is 8:00 am or the earliest time setup in all of your calendars that you are using for that project.

now for any finish milestones make sure that the TIME is 5:00 pm or the latest time setup in all of your calendars that you are using for that project.

now for any activities in progress meaning started but not complete make sure that the actual start date is  TIME is 8:00 am or the earliest time setup in all of your calendars that you are using for that project.

go to edit users preferences

click on the TIME Units tabs

under Durations Format change the decimals form 0 to 2.

now look for any activities that are in progress meaning started but not complete make sure that the remining duration is in whole numbers not fractions.

press F9 recalculate your schedule now a one day activity should only span one day NOT 2.

now when updating make any actual start times  8:00 am and actual finish times 5:00 pm and make sure that the remaining durations are in whole numbers.