Remove the duplicate values from a text rule input?

There maybe more elegant ways, but something like this:

a!localVariables(
  local!trimmed: fn!trim(ri!inputString),
  local!split: fn!split(local!trimmed, " "),
  local!lower: fn!lower(local!split),
  local!union: fn!union(local!lower, local!lower),
  local!result: a!forEach(
    items: local!union,
    expression: if(
      fv!isFirst,
      concat(fn!proper(fv!item), " "),
      concat(fv!item, " ")
    )
  ),
  fv!concat(local!result)
)

Notes:

1. fn!trim() removes all unnecessary spaces (in case you have more than one space between the individual strings) 
2. fn!split() breaks up the input strings into an array so they can be processed independently
3. fn!lower() makes all of the strings into lower case so that, for example, all "Hello" items are the same
4. fn!union() removes duplicate strings from the array
5. the a!forEach() allows us to treat the very first item in the array differently from the others, so we can use fn!proper() to capitalize the first letter for just the first string. For all items we add single space so that...
6. ...we can then concatenate all of the items in the array to a single output string 

hi stewart, is there an guide for fn! functions somewhere ? not all functions are available as fn! but is there any simple rule like all functions without a! or a page to explain them. 
Just curious.... 

Hi AKASH T,

can you try this one

a!localVariables(
local!string:union(split(lower("Hello hello Hello there there past pastures")," "),split(lower("Hello hello Hello there there past pastures")," ")),
substitute(tostring(local!string),";"," ")
)

There are a couple of resources I typically use:

In the expression designer interface you can access this list as follows:

There is also online documentation:

https://docs.appian.com/suite/help/21.1/Appian_Functions.html

(if you're not on 21.1 then you can navigate to https://docs.appian.com/ and then choose your version)

Works well But the result will be displayed in lower case. 

i was looking for online documentation :) 
I know how to search for specific function at the documentation (appian.com). My question targeted towards fn! functions specifically. we have a list of several functions provided by appian. Are all of them available as fn!

functions?

my code would have looked something like this:

a!localVariables(
  local!trimmed: trim(ri!inputString),
  local!split: split(local!trimmed, " "),
  local!lower: lower(local!split),
  local!union: union(local!lower, local!lower),
  local!result: a!forEach(
    items: local!union,
    expression: if(
      fv!isFirst,
      concat(proper(fv!item), " "),
      concat(fv!item, " ")
    )
  ),
  concat(local!result)
)

i was looking for online documentation :) 
I know how to search for specific function at the documentation (appian.com). My question targeted towards fn! functions specifically. we have a list of several functions provided by appian. Are all of them available as fn!

functions?

my code would have looked something like this:

a!localVariables(
  local!trimmed: trim(ri!inputString),
  local!split: split(local!trimmed, " "),
  local!lower: lower(local!split),
  local!union: union(local!lower, local!lower),
  local!result: a!forEach(
    items: local!union,
    expression: if(
      fv!isFirst,
      concat(proper(fv!item), " "),
      concat(fv!item, " ")
    )
  ),
  concat(local!result)
)