if any body has apply they said if you proved a police certificate before you came to canada you dont need to poroved anther one any body has idea
If in doubt, follow the instructions; otherwise, yep, follow the instructions.
Notwithstanding blatantly erroneous information posted by others, which is disheartening, the instructions regarding police certificates are straight-forward.
The primary question is simple enough, and is stated in item 10. b) in the current application CIT 0002 (02-2019), where it states:
b) In the past four (4) years, were you in a country or territory other than Canada for 183 days or more in a row (since the age of 18)?
Note, the relevant time period is the previous FOUR (4) YEARS, not five.
If your answer to this item is "no" you need consider it NO further. No further information needs to be provided in item 10. b) and no police certificate needs to be submitted.
If your answer to this item is "yes" you list the country or countries in the chart. And, with some exceptions, you need to include a police certificate from that country or countries.
The exceptions are fairly easy. Basically there are two exceptions: (1) impossibility (cannot obtain a clearance), and (2) clearance provided during PR process suffices.
Impossibility: If it is not possible to obtain such a police certificate, that is one of the exceptions; in this event you give an explanation for why a police certificate cannot be obtained. Remember, IRCC deals with immigrants from nearly every country or territory in the world and is well acquainted with whether or not an individual in Canada can obtain a police certificate from this or that country. Fact that it is difficult or inconvenient, even very difficult, does not rise to the level of NOT being able to obtain a clearance. (But not being able to obtain a clearance without traveling to the country will, I believe, be a sufficient reason for not submitting one.)
Clearance provided during PR process suffices: There are various scenarios in which this exception might apply.
The "Note" in the form's instructions states the usual scenario in which this exception applies:
Note: If you were in your country or territory of origin immediately prior to becoming a permanent resident and landing in Canada and this time falls within this four (4) year period, you are not required to provide a police certificate. Please indicate this in the explanation box.
This is mostly clear. Some may quibble about its precise meaning, but the safe approach depends on that time, the 180 days or more in row, being BEFORE landing and becoming a PR. If that was the time in that country, before landing, no need to include a police certificate. State something in the explanation box like "time was in HOME country prior to landing" and that should suffice.
There is another type of scenario which fits this exception and that is described in Example 1 in the instructions "help" (click on ? mark in application to open this; comparable information is also published in the Instruction Guide); it states:
Example 1
You lived in France for one year (365 days) before you became a permanent resident 3 years ago. You did not travel to France after you became a permanent resident. You would answer "Yes" to the question and you would need to provide a police certificate from France if you did not provide one with your immigration application. If you provided a police certificate from France with your immigration application, tell us this in the box provided at Question 10b.
This exception is consistent with another instruction in Item 10b which says that a police certificate will suffice if it has been issued since the last time the applicant was in that country, even if it was issued more than six months ago.
By the way, ALL adult immigrants (or nearly all anyway) needed to provide a police clearance with their PR application, so no just the fact of having done that does NOT mean no police certificate is needed.
CAVEAT: IRCC can always ask for a police certificate later in the process even if the applicant was not required to submit one with the application. If it is a close call, be as accurate as possible, and if no police certificate is required do not submit one. But the prudent applicant may proceed to obtain one from the other country to have just in case IRCC later requests it.