Powershell Only Add to Array if it doesn't exist - arrays

Question

In PowerShell v2, I'm trying to add only unique values to an array. I've tried using an if statement that says, roughly, If (-not $Array -contains 'SomeValue'), then add the value, but this only ever works the first time. I've put a simple code snippet that shows what I'm doing that doesn't work and what I've done as a workaround that does work. Can someone please let me know where my issue is?
Clear-Host
$Words = #('Hello', 'World', 'Hello')
# This will not work
$IncorrectArray = #()
ForEach ($Word in $Words)
{
If (-not $IncorrectArray -contains $Word)
{
$IncorrectArray += $Word
}
}
Write-Host ('IncorrectArray Count: ' + $IncorrectArray.Length)
# This works as expected
$CorrectArray = #()
ForEach ($Word in $Words)
{
If ($CorrectArray -contains $Word)
{
}
Else
{
$CorrectArray += $Word
}
}
Write-Host ('CorrectArray Count: ' + $CorrectArray.Length)
The Result of the first method is an array containing only one value: "Hello". The second Method contains two values: "Hello" & "World". Any help is greatly appreciated.

Answer 1

To fix your code, try -notcontains or at least WRAP your contains-test in parantheses. Atm. your test reads:
If "NOT array"(if array doens't exist) contains word.
This makes no sense. What you want is:
If array does not contain word..
That's written like this:
If (-not ($IncorrectArray -contains $Word))
-notcontains is even better, as #dugas suggested.

Answer 2

The first time around, you evaluate -not against an empty array, which returns true, which evaluates to: ($true -contains 'AnyNonEmptyString') which is true, so it adds to the array. The second time around, you evaluate -not against a non-empty array, which returns false, which evaluates to: ($false -contains 'AnyNonEmptyString') which is false, so it doesn't add to the array.
Try breaking your conditions down to see the problem:
$IncorrectArray = #()
$x = (-not $IncorrectArray) # Returns true
Write-Host "X is $x"
$x -contains 'hello' # Returns true
then add an element to the array:
$IncorrectArray += 'hello'
$x = (-not $IncorrectArray) # Returns false
Write-Host "X is $x"
$x -contains 'hello' # Returns false
See the problem? Your current syntax does not express the logic you desire.
You can use the notcontains operator:
Clear-Host
$Words = #('Hello', 'World', 'Hello')
# This will work
$IncorrectArray = #()
ForEach ($Word in $Words)
{
If ($IncorrectArray -notcontains $Word)
{
$IncorrectArray += $Word
}
}