Solaris Remove duplicate lines within blocks of text

I work under Sun OS 5.10 I look for a shell script (using awk, sed, ... ?) for removing any duplicate lines within the same block of text in the input file. Here is a short example of the input file (which is a script for plantuml) : @startuml graph1.png object1 --> link1 link1 --> object2 object2 --> link2 link2 --> object3 object3 --> link3 link3 --> object1 object1 --> link1 @enduml @startuml graph2.png object4 --> link4 link4 --> object1 object1 --> link1 link1 --> object5 link1 --> object5 @enduml Here is what I'd like to get as results : @startuml graph1.png object1 --> link1 link1 --> object2 object2 --> link2 link2 --> object3 object3 --> link3 link3 --> object1 @enduml @startuml graph2.png object4 --> link4 link4 --> object1 object1 --> link1 link1 --> object5 @enduml Explanations: A block starts with @startuml and ends with @enduml In the first block, duplicate line to be removed is: object1 --> link1 In the 2nd block, duplicate line to be removed is: link1 --> object5